Full Pen Welding Symbol: How to Read CJP on Drawings

A fitter hands you a print for a 12-inch pipe spool. The symbol shows a V-groove with a small solid half-circle sitting on the far side of the reference line, and the tail just says “CJP.” The lead welder calls it “full pen” and asks whether you gouged the root for the back pass. If you had to guess what that half-circle means, this guide is written for you — because “full pen” is shop-floor language that shows up on thousands of drawings without ever being defined.

What “Full Pen” Actually Means on a Welding Drawing

The fastest way to get into trouble with a welding symbol is to treat shop-floor slang as if it were a specification. “Full pen,” “full penetration,” “100% pen,” and “CJP” all get used interchangeably in the trailer, at the weld bench, and in the foreman’s office. They all point at the same condition. On the drawing itself, though, only the standardized version counts.

“Full pen” appears nowhere in AWS A2.4:2020, the ANSI-approved standard that governs welding symbols in the United States. A tail note reading “full pen” tells a fabricator nothing that can be inspected later. A tail note reading “CJP” does. That gap between how we talk and how we specify is where most of the confusion in this topic comes from.

Shop language versus drawing language

What you hear in the shop	Standard meaning	Carries meaning on a drawing?
“Full pen,” “100% pen,” “full penetration,” “penetrating weld”	Complete Joint Penetration (CJP)	No. Not a term in AWS A2.4
“CJP” in the tail of the symbol	Complete Joint Penetration	Yes, with a recognized callout
“PJP” or “partial pen”	Partial Joint Penetration	Yes, when dimensioned
“Melt-through,” “back weld,” “with backing”	Graphical symbols	Yes, when drawn as symbols

There are five recognized ways to state a full penetration requirement on a drawing: the letters CJP in the tail, an undimensioned groove weld symbol, a groove weld symbol paired with the melt-through symbol, a groove weld symbol paired with the backing bar symbol, and a groove weld symbol paired with the back weld symbol. All five are covered in detail further on. The important point here is that nothing else on the drawing carries the same contractual weight.

The formal definition

Caltrans states it plainly in Bridge Design Details, Chapter 11.3 (February 2025). Complete Joint Penetration is a groove weld that extends completely through the thickness of the components joined and should have the same strength as the components being joined.

Partial Joint Penetration, or PJP, is the same thing short of the finish line: a groove weld that does not extend completely through the thickness of the components joined. On a drawing, PJP is always dimensioned, and its effective weld size appears in parentheses to the left of the groove symbol. An undimensioned groove symbol means the weld goes all the way through.

What a CJP callout does and does not promise

A CJP callout specifies a weld condition. It does not specify preheat, interpass temperature, filler metal, welding process, a qualified procedure, or any inspection method. Those come from the governing code (AWS D1.1 for structural work, ASME Section IX for pressure equipment, API 1104 for pipelines) and from the Welding Procedure Specification. Process and consumable selection is a separate decision based on base metal, thickness, and position, which is why our guides to position welding rod selection and to aluminum MIG welding matter just as much as the symbol once the CJP condition is set.

So a drawing that says “CJP” in the tail and nothing else is a contract with a gap in it. The weld condition is defined, but the procedure and acceptance criteria are not.

Where the rules come from

AWS A2.4:2020, Standard Symbols for Welding, Brazing, and Nondestructive Examination, is the current US standard: eighth edition, approved by ANSI on January 9, 2020, superseding AWS A2.4:2012. The standard itself has a long lineage. It came into existence in 1976 by consolidating AWS A2.0, first published in 1947, and AWS A2.2, first published in 1958, into a single reference for symbol interpretation. The current edition is available through AWS, and the ANSI blog covers the changes that made 2020 a real update rather than a reissue.

Internationally, the parallel document is ISO 2553. The symbol set is largely the same, but the placement convention is inverted, which is one of the most common ways a drawing gets misread. We come back to that in the troubleshooting chapter.

The measuring stick: effective throat

Every symbol is ultimately about one number, and that number is the effective throat: the shortest distance from the joint root to the surface of the weld. It is not the same as a fillet weld’s leg dimension, and it is not the visible cap height. For a CJP groove weld, the effective throat equals the full thickness of the joint, because the weld reaches the root and fills the gap. For a PJP weld, the effective throat is less, so it gets a number. This is why dimensioning a groove symbol immediately changes the meaning of the callout, and why the definition is the measuring stick for every rule that follows.

Anatomy of a Welding Symbol: Arrow, Reference Line, and Tail

Before a symbol can tell you it means full penetration, you have to be able to read it. A welding symbol is a compressed sentence built from a few strokes: an arrow, a horizontal reference line, and a tail. Two of those three are mandatory. Everything else in this guide, from groove shapes and sizes to process letters, is optional detail layered onto that base.

The three elements, and the two you cannot skip

The arrow is the leader line that connects the symbol to a location on the drawing. The reference line is the straight horizontal line that carries the weld information. The tail is the optional flag at the far end that holds general notes. Per the ANSI write-up on the A2.4:2020 revision, only the reference line and the arrow are required; the rest of the standard exists so a drawing can add welding information in a controlled, readable way.

The arrow and reference line are the subject of the sentence, and everything else is a modifier. A symbol with a bare reference line is technically complete, yet tells the shop almost nothing about what to build.

Arrow side and other side

Two terms cover every joint in the world. The arrow side is the side of the joint the arrowhead points to. The other side is the face directly opposite it. Once you can name those two faces, you can read any symbol, because the position of the symbol on the reference line tells you which face it belongs to.

Symbol position	Applies to
Below the reference line	Arrow side
Above the reference line	Other side

The National Board of Boiler and Pressure Vessel Inspectors calls this system “a useful system or undecipherable hieroglyphics,” depending on whether the reader knows the rule. The rule never changes: the position of a symbol on the reference line, not the direction the arrow points, decides which side it governs. It works the same whether the arrow attaches to the left end or the right end of the reference line. An arrow on the right is a mirrored layout, not a different convention, and arrow-side information still goes below the line.

Pointing the arrow at the right line

Good drafting here saves a lot of arguing later. The arrow must point to a line that conclusively identifies the joint, since that link is the only thing tying the symbol to the physical connection. AWS recommends the arrow point to a solid, visible object line. It may point to a dashed hidden line, but only where that is the only line available, and then the reader is warned to be certain of the location. The current AWS A2.4:2020 preview carries the same thinking forward. A symbol aimed at the wrong line is not a clarification issue. It is a weld made in the wrong place.

The tail: where everything else goes

The tail is the greater-than or less-than shape drawn at the end of the reference line opposite the arrow. It is the catch-all for information with no other home. In practice you will see:

• Process designation letters such as SMAW, GMAW or FCAW, which fix the process to be used. Our guide to why SMAW still wins goes deeper on the stick process.
• A WPS number, pointing to the qualified welding procedure the shop must follow.
• Notes such as CJP, FW or UW that state a condition rather than a dimension.
• A reference to a shop standard or detail sheet, offloading routine groove geometry, contour and finish onto a separate document.

One change worth knowing: A2.4:2020 no longer supports the old flash and upset welding symbols. Those joints are now specified with a plain reference line and arrow, with the process identified by FW or UW in the tail.

Multiple arrows and the jog

Two smaller features round out the grammar. Where the same weld detail applies at more than one location around a closed shape, several arrows can be tied to a single reference line, and each arrowhead identifies one of the identical welds. Where two members meet and the drawing needs to be specific, a jog (a break or bend) in the arrow identifies which member receives the edge preparation, so nobody guesses who is doing the beveling.

The Five Ways to Specify a Full Penetration Weld

Ask five competent designers to call out complete joint penetration and you will get five different drawings. That is not sloppiness. It is because AWS A2.4 recognizes several legitimate ways to state the same requirement, and the National Board of Boiler and Pressure Vessel Inspectors lays them out as a menu rather than a single mandatory form. Bill Green, retired professor emeritus of welding engineering at The Ohio State University, walks through the options the way he teaches them: pick the callout that states your intent and leaves the fabricator room to execute it.

Method 1: Write “CJP” in the Tail

The simplest and most common approach is to place the letters CJP in the tail of the welding symbol. This states the condition without dictating a groove shape, a process, or a backing arrangement. It is the right call when the drawing is governed by a code that already defines what complete joint penetration means and how it must be verified. What it does not do is tell the shop how to get there.

Method 2: A Single Groove Symbol, or Matching Double Groove Symbols

Here the symbol itself carries the requirement. A single groove weld symbol drawn on the reference line, or a pair of matching groove symbols on opposite sides of it, means complete joint penetration as long as no depth-of-bevel dimension and no weld size dimension appear. The dimension stack is the tell. An undimensioned groove symbol means the welder may choose any of the groove shapes from the standard family, and the joint still has to be made full penetration.

Method 3: The Melt-Through Symbol

This is the method for a single-sided weld where the root must show visible reinforcement on the back side. The melt-through symbol goes opposite the groove weld symbol, and the height of the required root reinforcement may be specified by adding the appropriate dimension to the left of the melt-through symbol. If you are welding thin-wall aluminum or stainless work where blowing through the root would ruin the part, this is the callout that keeps the welder honest, and it is why aerospace TIG welding procedures lean on reinforcement control rather than on sound alone.

Method 4: The Backing Bar Symbol

Place the backing bar symbol opposite the groove weld symbol and you have told the shop a permanent backing member is required to support the root. The backing supplies the molten metal support that lets the groove weld achieve full penetration in a single pass.

Method 5: The Back Weld or Backing Weld Symbol

Drawn opposite the groove symbol, the back weld is madeafter the groove weld and assures full weld metal through the joint, a point made in Goodheart-Willcox’s Interpretation of Metal Fab Drawings. The related backing weld is a bead-type weld placed on the opposite side of the regular weld, typically to prevent melt-through of the root. Both demand access to the far side, and a back weld usually means gouging the root first, which is a good reason to be comfortable with air arc gouging before you take the job.

A Related Callout: Consumable Inserts

An open square placed on the reference line opposite a groove weld symbol indicates a consumable insert is required. The AWS consumable insert class must then be referenced in the tail of the symbol. This is common in thick-section pressure work and heavy fabrication where the insert keeps the root from burning through.

Method	What It Guarantees	Typical Application	Watch-Out
CJP in the tail	Code-level full joint strength, no method fixed	General code-governed drawings	Specifies no process, groove, or root condition
Single or matching double groove symbol, undimensioned	Complete penetration with any standard groove geometry	Most CJP butt joints in plate and pipe	Any added dimension changes the meaning
Melt-through symbol opposite groove symbol	Visible root reinforcement of the specified height	Single-sided welds with an inaccessible far side	Reinforcement must be flat; not a back weld
Backing bar symbol opposite groove symbol	Root support for a permanent backing member	Plate and heavy fabrication with permanent backing	Crevices, harder UT, added fit-up
Back weld or backing weld symbol opposite groove symbol	Full weld metal through the joint	Both sides accessible	Extra heat, distortion, and gouging work
Consumable insert (related)	Root support and burn-through resistance	Heavy sections and pressure equipment	Insert class must appear in the tail

That last point trips up more readers than any other. Partial joint penetration is specified by adding the depth-of-bevel dimension and the required weld size, in parentheses, to the left of the groove weld symbol. A single number means the designer is calling a specific depth. A number in parentheses next to the depth-of-bevel means the effective weld size the joint must actually achieve. Once either appears, the full-pen assumption is gone and the weld is judged on partial penetration rules.

Pick the callout that matches the design intent, and resist the urge to add helpful-looking dimensions. Every dimension you add to a groove symbol is a specification you are handing away.

Groove Weld Symbols Decoded: Square, V, Bevel, U, J, and Flare

Before you can read a full penetration callout, you need to recognize the groove symbol on the reference line. These symbols are almost pictorial: a square groove looks like a square, a V groove looks like a V, a J groove looks like a J. The Open Oregon guide to reading metal fabrication drawings is the best free walkthrough of the family.

Square Groove

The square groove symbol looks like a square bracket, or a fat I-beam standing on the reference line. The joint faces are prepared square to each other, either tight together or with a small root opening. These welds are common in thin material and tubing, where a bevel adds cost and heat without adding penetration.

Single V and Single Bevel

The single V symbol is the chevron everyone pictures. The single bevel symbol is the same shape but asymmetric, so it looks like a flag, or a half V leaning to one side. A V groove takes weld metal from both sides, while a bevel groove gets reinforcement from one side only and usually pairs with a backing bar or a second weld.

Double V and Double Bevel

The double V symbol is two chevrons pointing at each other with a vertical line between them. The double bevel is the asymmetric version, two half V shapes meeting at the root. Weld both sides for better heat and distortion control, which is why they turn up in bridge and heavy plate work.

U and J Grooves

U and J are the narrow throat preparations used on thick plate. The U symbol looks like a U with rounded bottom corners, and the J looks like a hook. Both are expensive to cut, so they are specified when the joint must be made in very few passes, when filler metal cost matters, or when the design demands a specific root face and radius.

Flare-V and Flare-Bevel

Flare grooves are used when one member is flared open rather than cut with a straight edge. The flare-V symbol combines a V shape with a bell-like flare at the base, and the flare-bevel uses the flare on one side only. They are common on tubing 1/4 inch and thicker and in sheet metal, since the flare legs supply much of the required filler metal, and they are the standard structural detail for round solid, pipe, and square or rectangular tube members. The official AWS weld symbol chart shows all of these side by side with the supplementary symbols, so print it and keep it in your hard hat folder.

The Dimension Stack Around a Groove Symbol

Groove symbols rarely stand alone. The dimensions that tell the fabricator how to prepare the joint sit on either side of the symbol, and mixing them up builds the wrong weld fast.

• Depth of bevel (groove depth): left of the groove symbol, how deep the prepared groove goes into the joint.
• Effective weld size: also to the left, in parentheses when it accompanies a depth of bevel. This is the strength-carrying dimension of the weld.
• Root opening and root face: the gap between the prepared faces (root gap), and the flat unmelted land left at the root when groove depth is less than full plate thickness.
• Included groove angle: the angle in degrees.
• Weld length: to the right of the symbol, typically in inches with a pitch if the weld is intermittent.
• Contour and finish symbols: below the reference line, describing the face profile and surface condition of the finished weld.

The Rule That Matters Most: No Weld Size Means Full Penetration

If no weld size is shown on a groove weld, the weld shall be complete joint penetration. AWS states this plainly, and it is the rule that turns a bare V symbol into a full penetration instruction. Want partial penetration? You have to dimension it deliberately.

The mirror image matters just as much. When a groove depth is shown without a matching weld size, the weld shall not be less than the depth of preparation, so it is not a penetration callout. Add a depth of bevel dimension and you have changed the symbol’s meaning, exactly the trap described in the welding symbols tutorial on reading a full print.

Groove Angle Versus Bevel Angle

Angle callouts are where AWS and shop habit collide. A groove angle runs face to face across the whole groove, so a 60 degree groove is 60 degrees total. A bevel angle is one half of a V groove, so a 30 degree bevel edge yields a 60 degree groove once the members are assembled. Check which one the drawing means before you set your torch travel.

Full Pen in Structural and Bridge Steel: CJP Versus PJP

Reading a symbol is one thing. Deciding which symbol the engineer meant is another. That decision lives in the code, not on the drawing.

For building frames, bridge girders, and anything else that must carry load, the governing document in the United States is AWS D1.1/D1.1M, Structural Welding Code — Steel. Referenced on nearly every structural steel erection package in North America, it is where the line between complete joint penetration and partial joint penetration becomes a rule rather than a preference.

What D1.1 Actually Governs

D1.1 organizes structural steel welding into a few big blocks. Clause 9 is the design chapter, and it is where the symbol language lives:

• Clause 9.4, Symbols, governs welding symbols on structural drawings, including how the reference line, arrow, and tail apply to structural joints.
• Clause 9.5, Weld Design, addresses the design of welded joints, including allowable stresses and effective throat requirements.
• Table 9.1 lists the prequalified groove weld details: joints AWS has already evaluated for strength, so a shop welding them with the specified process, filler metal, and geometry needs no separate qualification test.
• Annex I, which is normative, covers Symbols for Tubular Connection Weld Design. This is the part most people have never opened, and it is where round, box, and pipe connections get their own symbol vocabulary.

The 2015 revision of D1.1 did housekeeping in that annex: Annex I definitions for the symbols l2, rm, and tw were revised, and a new symbol rw was added, matching corresponding changes in Figure 9.6.

Caltrans Calls It CJP or PJP. Nothing More.

Now look at how a public agency with thousands of fabrication shops to manage handles the same problem. The California Department of Transportation Bridge Design Details give explicit guidance, a good example of how a modern owner draws this:

• When a CJP or PJP groove weld is called out, do not detail the specific groove preparation.
• Do not call out a specific weld type such as a double-V or a square groove.
• Point the arrow to the joint and simply call out CJP or PJP.

The reasoning is straightforward once you say it out loud. The fabrication shop owns the groove detail, because it knows its welding stations, its fit-up and distortion control, and its sequencing. The engineer owns the load path. Design the load path, let the shop design the groove.

The same principle shows up in how the symbols are read. The CJP symbol does not denote weld size at all. The PJP symbol does, carrying two numbers: E1 for the other side and E2 for the arrow side.

The PJP Prohibition List

Because PJP is a weaker condition, the code puts boundaries around where you may use it. Per Caltrans Memo to Designers 12-5, PJP groove welds are:

1. Prohibited in butt joints, except those conforming to AASHTO/AWS D1.5 Article 2.17.3, which covers connections or splices in compression members with milled joints.
2. Prohibited where the applied tensile stress is normal to the effective throat, that is, where the load pulls directly across the throat of the weld rather than shearing along it.

There is also a floor on how small a PJP weld can be. AASHTO/AWS D1.5 Table 2.2, as Caltrans cites it, sets the minimum effective weld size for PJP groove welds at 1/4 inch (6 mm) for a thicker-part thickness under 3/4 inch (20 mm), and 5/16 inch (8 mm) above 3/4 inch (20 mm). Effective weld size follows the depth of bevel, with or without a 3 mm (1/8 inch) deduction depending on how the joint is prepared and welded.

Why Guessing PJP Is So Expensive

Here is the practical consequence of reading a callout wrong. If the design intent was load-carrying capacity and the shop welds a shallower PJP instead of a full CJP, the joint may look fine on the print and still be the weak link. It then fails an ultrasonic test, gets flagged in an NDE report, and every remediation path costs money:

• Excavate the weld and re-weld to full CJP, meaning gouging, re-prepping, re-fitting, and re-welding in a member already connected to the rest of the frame.
• Reject and replace the member entirely.
• Engineer a reinforcing detail, which is slow and consumes design time on both sides.

None of that means CJP is always right. On a lightly loaded gusset, a CJP weld can add heat input, cost, and distortion for strength nobody needs. Match the callout to the actual demand, then weld exactly what the callout says.

For pipe, tank, and pressure equipment work, the same logic applies under different codes. Moving from structural steel to process piping or vessel work shifts you to a different family of qualification and acceptance criteria, and not every assumption you carried over from D1.1 will hold. Structural pipe and pressure pipe are not the same job, even when the groove symbol looks the same.

Next comes where full penetration is not a choice but a legal and safety requirement, and how the symbol changes once you are inside a pressure boundary.

I’ll write section 6 of the article now, covering full pen welding on pipe, tanks, and pressure equipment.

Full Pen on Pipe, Tanks, and Pressure Equipment

In pressure equipment, penetration stops being a style choice. A vessel wall, a tank seam, or a pipeline joint that leaks does not fail gracefully, so complete joint penetration becomes the default assumption behind nearly every detail in these industries. The difference from structural steel is that nobody gets to argue about groove geometry on the shop floor. The codes set the expectation, the welding procedure qualifies the welder, and the inspector verifies the result with radiography or ultrasonic testing.

The Codes Behind the Symbols

For ASME work, Section IX of the ASME Boiler and Pressure Vessel Code covers welder and welding operator qualification, while Section VIII governs how vessel components are built and joined. A “CJP” in the tail on a pressure vessel drawing is shorthand for a joint evaluated against those requirements, not a free-form shop instruction. Outside ASME, AWS A2.4 supplies the symbol language, and AWS D1.1 clauses on visual inspection (6.9) and surface methods (6.10) fill in inspection expectations.

Material family drives the rest. Stainless falls under AWS D1.6/D1.6M and aluminum under AWS D1.2/D1.2M, each with its own qualification ranges, filler metal rules, and inspection requirements. Corrosion-resistant service has its own filler metal headaches, covered well in the alloy 20 welding procedures guide. Aluminum resists several of the assumptions that come with CJP work, so read the aluminum MIG welding guide before touching a qualified procedure. Pipeline work runs on API 1104, which pairs with ASME Section IX to govern pipe welding in the shop and the field.

Choosing How to Achieve Full Pen on Pipe

Three variables decide whether a pipe joint can be welded from one side or two, and each one changes what symbol goes on the drawing.

• Root opening or backing. If two welders can reach either side of a spool or a flange, a double groove weld gives the tightest control of heat and distortion. If access is one-sided, the shop needs backing or a melt-through requirement.
• Consumable inserts. An open square placed on the reference line opposite the groove weld symbol specifies a consumable insert, with the insert class referenced in the tail of the symbol, per AWS A2.4:2020. The root pass then fuses to a placed filler rather than directly to the parent metal, a common trick on stainless and on stubborn root passes in heavy wall.
• Melt-through. On a single-sided weld where the process must not blow through, the melt-through symbol opposite the groove symbol calls for visible root reinforcement on the back side, with any required reinforcement height dimension placed to the left of the melt-through symbol.

Scarf Joints and Flare Grooves

Not every tube joint gets a V. The National Board describes the scarf weld as intended for lighter-gauge material in small-diameter piping, tube-to-tubesheet applications, and attaching seats in smaller valves, and states that it is intended primarily for brazed joints rather than heavy fusion welding. Separately, flare-type groove welds are used extensively where solid rounds, piping, and square and rectangular tubing are employed for structural purposes rather than to withstand internal pressure. That is why you see flare-V and flare-bevel symbols on handrail, stair, and rack drawings, and almost never on a pressure boundary.

Backing: Steel, Consumable, or a Back Weld

Backing is where full-pen detailing gets expensive. A permanent steel backing bar is fast and lays down a smooth root, but it creates a crevice that traps corrosion and complicates inspection. Consumable backing, ceramic or flux-cored, gives a better root profile and makes ultrasonic work easier, at the cost of more consumable handling. A back weld removes the need for either and is the cleanest answer whenever both sides are reachable.

Weld-All-Around and Inspection Reality

On tanks and vessels, the weld-all-around circle at the junction of the arrow and reference line tells the shop the seam runs continuously around the joint. Tank shell courses, pipe rings, and nozzle set-ins all depend on that instruction, and a missing circle on a circumferential seam is an expensive gap nobody wants to explain after the fact.

On inspection, UT is the default for full-pen CJP in pipe and plate because it maps root penetration directly instead of inferring it from a surface reading. Radiography remains the fallback for pipe that cannot be scanned and for many ASME-mandated examinations.

Weld-All-Around, Field Weld, and Multiple Reference Lines

While a groove symbol tells you the joint preparation and penetration requirement, three supplementary symbols dictate how a full-pen weld is executed, not how much penetration is called for. These are the weld-all-around, the field weld, and multiple reference lines. All of them are defined under AWS A2.4:2020, specifically within Clause 6.8 (Field Weld Symbol), Clause 6.9 (Extent of Welding Denoted by Symbols), Clause 6.11 (Weld-All-Around Symbol), and Clause 6.12 (Tail of the Welding Symbol). Additional guidance can be found within Clause 8.1 for fillet weld size and length, and Clause 8.2 for the intermittent fillet weld symbol.

The Weld-All-Around Symbol

The weld-all-around symbol is a small circle placed at the junction of the arrow and the reference line. This simple addition means the weld is to run continuously around the entire member or joint, with no break in the weld unless otherwise specified. This is especially vital for pipe rings, tank shell seams, hollow structural sections, and any circular or closed joint that would leave a weak point if stopped short. For tubular fabrication such as aluminum boat hull joints, this continuous seal is often the difference between a sound joint and a leak path.

The Field Weld Symbol

Not every weld can be made within the controlled environment of a shop. When a weld must be performed in the field, a flag is added to the tail of the welding symbol. This flag designates a field weld, and it drastically changes the game for the crew. Access can be limited, position may be vertical or overhead, weather protection must be accounted for, and the NDE method selection may need to be adjusted to suit on-site conditions. For this reason, a field weld callout should never be treated as an afterthought. It is a logistical change as much as it is a welding change.

Multiple Reference Lines: Sequencing the Work

One of the most powerful, yet overlooked, tools on a drawing is the use of multiple reference lines. When two or more reference lines connect to the same arrow, the reference line closest to the arrow specifies the first operation, with the sequence reading away from the arrow. From there, each subsequent line denotes the next operation in order.

This allows designers to sequence welds with precision. For example, a groove weld can be called for on the first line, with a back weld specified on the second to be completed after it. Even more importantly, operations other than welding, including nondestructive examination, can be specified by a multiple-reference-line symbol. This allows an inspector to know exactly when a hold point, such as an examination, is required during fabrication.

As part of the updates made within AWS A2.4:2020, the standard now permits multiple subreference lines to designate a groove weld extending around a joint where there is no clear point at which the joint transitions from one joint type to another. This removes ambiguity for complex, wrapped joints without having to over-detail the assembly.

At the end of the day, none of these symbols change the CJP requirement itself. They change the plan to achieve it. Knowing whether a full-pen weld needs to wrap around, be completed in the field, or follow a strict sequence could be the very thing that keeps your weld sound, your schedule intact, and your rework to a minimum.

Seven Ways to Misread a Full Pen Symbol

Most expensive welding mistakes start with a good weld of the wrong thing. These seven misreads drive most rework orders, NDE rejections, and RFQs after a full-pen callout.

1. Flipping Arrow Side and Other Side

AWS and ISO disagree on where arrow-side information lives. Under AWS A2.4:2020, arrow-side data sits below the reference line and other-side data above. ISO 2553 does the exact opposite. A shop applying AWS habits to an ISO drawing welds the back side onto the wrong face of the joint.

2. Confusing Back Weld with Backing Weld

Both carry a small half-circle opposite the groove symbol and can look like the same call at a glance. They are not. A back weld is a full structural weld on the far side of the joint, expected to carry load. A backing weld only supports molten metal from behind. AWS shows both with nearly the same graphic and distinguishes them by arrow side; ISO 2553 gives them separate symbols. Get it wrong and you skip a required weld or grind out a structural one. Back gouging a root before the second pass is a third thing entirely, done with air arc gouging.

3. The Depth-of-Bevel That Silently Turns CJP into PJP

Put a depth of bevel (depth of groove) dimension to the left of a groove symbol and the callout stops being full penetration. Nothing warns you: the symbol still looks like a V and the tail may still say CJP. Caltrans BDD 11.3 shows the difference, with the effective size for a PJP weld in parentheses to the left of the groove symbol.

4. Melt-Through Missing Where It Belongs

Melt-through tells the shop the joint will be fused completely through, leaving root reinforcement on the back. Omit it on a single-sided weld needing visible root reinforcement and the inspector has nothing to check against. Add it where a permanent backing bar is specified and you get redundancy at best, contradiction at worst: the backing already supports the root, and a reinforcement height has nothing left to measure.

5. Reading Fillet Size and Pitch Wrong

Not every symbol on the reference line is a groove weld. The number left of a fillet symbol is the leg, not the effective throat. AWS defines fillet weld size as the leg dimension, while ISO allows size as either leg or throat. Since an equal-leg fillet throat is roughly 70 percent of the leg, the same fillet differs by nearly 30 percent between standards. Pitch is worse: for intermittent fillets, AWS pitch is the center-to-center distance between segments on one side of the joint, while ISO pitch is the clear distance. Keep the AWS weld symbol chart and the ISO sheet side by side; never mix conventions.

6. Mismatched Symbols on a Double Groove Weld

A double groove weld is full penetration only when both sides work together. Put a V on one side and a square groove on the other, or leave dimensions on one side only, and you have put an unknown into the joint. Open Oregon’s guide to metal fab drawings puts it bluntly: for a double groove weld with matching dimensions on both sides, dimensions must be shown on both sides. One side missing means one side unknown.

7. CJP in the Tail with Nothing Behind It

CJP alone in the tail states the condition you want and nothing about how to achieve it: no governing code, no WPS, no filler metal, no preheat, no examination. Symbols can also specify nondestructive examination, including method, frequency, and extent, so leaving those off is a choice. With no procedure specified, the shop decides what it thinks reasonable, and many reach for the process they know best, such as SMAW for full-penetration work on heavy plate.

Don’t Dictate the Groove, and Don’t Guess Either

One more trap: specifying a double-V or square groove when the intent is simply “full penetration.” Caltrans makes the opposite call on bridges, specifying CJP and PJP generically without naming a groove type, because the fabricator is best placed to choose the groove detail based on equipment and distortion control. Lock in a groove and you may lock in a process the shop cannot run. Leave it open and you must state the condition clearly or you get PJP by accident.

When shop equipment and procedure are not yet known, the answer is escalation, not interpretation. The National Board recommends submitting assembly drawings to engineering for final approval, with welding symbols containing all pertinent details, when CJP is in the tail and shop equipment is not known. A single reference line also fails to specify operation sequence when both sides are welded; multiple reference lines are required.

If a symbol is ambiguous, ask in writing before you cut metal. A clarification request costs an email. A misread symbol costs a spool, a rejected radiograph, and a delay.

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Proving Full Penetration: Inspection and NDE on CJP Welds

A CJP callout is a promise: the finished joint carries load across the full thickness of the components it joins. Somebody has to verify that promise, and the symbol tells the inspector what to look for. Get it wrong and you learn about it with a grinder.

Inspection Comes in Layers

The hierarchy runs from broad and cheap to targeted and expensive. Under AWS D1.1/D1.1M, clause 6.9 is Visual Inspection and clause 6.10 is Penetrant Testing (PT) and Magnetic Particle Testing (MT):

1. Visual (VT). Weld profile, size, length, cracks, undercut, arc strikes, and gross defects visible without magnification.
2. Surface methods (PT and MT). Fine cracks and surface-breaking flaws. PT works on any clean, nonporous surface; MT only works on ferromagnetic material, so aluminum and austenitic stainless lean on PT and visual. Clean, oxide-free surfaces are a precondition, which is why our aluminum cleaner for welding guide matters.
3. Volumetric methods (UT and RT). Internal flaws. UT maps root and sidewall fusion directly and is the default for full-pen butt welds in plate and pipe. RT produces a film image of the joint, which is why aerospace, for one, leans on it heavily, as our aerospace TIG welding standards guide shows. RT is slow, needs both sides, and dislikes backing bars.

When a drawing calls for NDE, method, frequency, and extent come from the nondestructive examination symbols in Part C of AWS A2.4 (see the A2.4:2020 standard page). The examination letter sits below, above, or centered on the reference line: arrow side, other side, or no side preference. That is the same grammar as the weld symbol, so a drawing mixing AWS and ISO conventions can point a technician at the wrong face of the joint. The 2020 revision also changed how size or strength may be designated for spot and seam welds.

Incomplete Penetration Is the Usual Culprit

When a full-pen weld is rejected, the finding is almost always incomplete penetration at the root, also called lack of root fusion or a cold lap at the root. The AWS Inspection Trends case study on CJP welds shows the pattern: recurring single-bevel corner joint failures traced to a blocked root gap and incomplete sidewall fusion. The root opening was too tight, slag, oxide, or grind marks sat at the root as the puddle formed, or travel speed outran the arc so the metal never melted into the base metal. From the top it looks fine, so visual inspection alone never catches it.

Your Callout Decides the Inspection Strategy

• Melt-through symbol. Root reinforcement on the back side must be continuous, flat, and of the height dimensioned on the drawing, and the welder has to see the back of the joint, so access is planned before the arc starts.
• Backing bar symbol. Permanent backing is fast and gives a smooth root, but the junction with base metal can form a crevice UT must evaluate, and it blocks the radiation path radiography needs. If the print does not resolve inspection, ask first.
• Back weld or matching double groove weld. Two surfaces to interrogate helps UT, but it creates a second set of fusion problems. Root passes must be cleaned and verified before the second side is welded.

What the Inspector Can and Cannot Measure

A CJP symbol with no dimensions leaves groove geometry wide open, so the welder may have used any acceptable preparation and the inspector falls back on code acceptance criteria. Where you did specify something, it gets measured: effective throat where dimensioned, root reinforcement height and contour on a melt-through joint, and the general shape and size of the weld.

Sequencing is another gap. As the National Board of Boiler and Pressure Vessel Inspectors notes, the welding symbol does not specify that welding from one side is completed before welding starts on the opposite side. Neither you nor the inspector can assume a back pass followed a clean root unless the drawing says so with multiple reference lines.

Preheat, Interpass, and the Root

Some root failures are metallurgical, not procedural. On thick or heavily restrained joints, insufficient preheat and a low interpass temperature slow cooling, trap hydrogen, and encourage cracking, while poor heat input or a cold, dirty root contributes directly to lack of fusion. That is why the WPS carries preheat and interpass limits and why inspectors ask for temperature logs.

When the Inspection Fails

Stop. Isolate the joint, do not weld around it, and bring the engineer in before anyone touches a grinder. A full-pen joint that failed for lack of fusion may have to be gouged out and rewelded entirely, and an already heat-treated joint may need post-weld heat treatment plus qualification of the repair welder. If a drawing ambiguity allowed a shallower weld, the original symbol, the WPS, and the inspection report belong on the table.

A Practical Checklist for Getting Full Pen Right the First Time

Every expensive misunderstanding starts the same way: someone glanced at a symbol for ten seconds and welded what they assumed it meant. A full-pen joint is not hard to specify, just easy to leave ambiguous. Here is the compressed checklist for the two people who carry the result.

Designers: decisions that prevent rework before fabrication

1. Call out CJP generically. Specify the condition, not the groove shape. Caltrans guidance is explicit that CJP and PJP should be noted without detailing a specific groove such as double-V or square, because the fabricator is best positioned to choose the groove detail based on equipment and capacity, which also limits distortion. See Caltrans Bridge Design Details, Section 11.3.
2. Add the melt-through symbol plus a root reinforcement dimension whenever the joint is welded from one side. Without both, the inspector has no condition to accept and “full” at the root becomes opinion.
3. Use the backing bar symbol only when permanent backing is genuinely acceptable and the inspection strategy is agreed with the shop in advance. Backing changes what UT sees and what corrosion can hide in.
4. Use multiple reference lines when sequence matters, such as groove weld first and back weld second, or NDE called at a specific stage of the assembly.
5. Reference the governing code and the WPS in the tail, with the examination method, frequency and extent. AWS A2.4:2020 establishes how welding, brazing and NDE information is specified by symbols, and a symbol alone will not tell a shop which procedure applies.
6. State the symbol convention on the drawing. Put “AWS A2.4 symbols” or “ISO 2553 symbols” in the general notes and title block, then never mix them on a sheet. A2.4 includes symbol designs in both inches and millimeters (Annex A and Annex C of the 2007 edition), cutting conversion errors on mixed-unit drawings.

Shops and fabricators: four rules that protect the margin

• Read the symbol before you estimate. Price the weld, not the joint. A CJP callout with a backing bar and melt-through requirement is a different job than a PJP, and the quote should reflect it.
• Check the convention against the title block before assuming which side of the reference line is the arrow side.
• Ask in writing when a symbol is ambiguous, and attach the answer to the assembly. Verbal clarifications do not survive a change order.
• Never substitute a shallower weld for a full-pen callout. If the procedure, fit-up or access will not deliver CJP, stop and escalate rather than downgrading the joint.

The National Board of Boiler and Pressure Vessel Inspectors makes a related point: there is considerable advantage to developing a shop standard for welding symbols, since it establishes the details that apply to all normal or standard joints and reduces interpretation errors. Joint geometry varies from manufacturer to manufacturer because cost is tied to the equipment in use.

Cost and distortion: what each approach really buys you

Approach	Gain	Cost
Single-sided groove weld with melt-through	Fastest, one side prepared	Root left to grind; blow-through means rework
Double-sided groove weld	Better heat control, less distortion	Doubles prep with gouging and a second pass
Permanent steel backing bar	Quick, smooth root	Complicates UT, hides crevices, corrosion risk
Consumable backing	Best root profile, easier UT	Higher consumable cost, more setup

Consumables and process choices, without the sales pitch

Filler metal must match the joint, the process and the code. On thicker plate, FCAW suits full-pen work for its deep root penetration, but stick-out and slag control get harder on a hot, dirty root pass. On thinner material, GMAW gives the clean root most shops want without the heat load of stick, provided the joint is clean and the shielding gas is right. For field repairs and awkward fit-up, SMAW still earns its place, which is why so many shops keep the case for stick welding on hand. The all-position welding rod guide is a practical starting point for crews new to filler selection, and the welding equipment guide covers a standard gas and equipment package.

Bottom line: “Full pen” is a condition, not a process or a promise of workmanship. The symbol is the contract, and five callouts (CJP in the tail, an undimensioned groove symbol, melt-through, a backing bar, or a back weld) are the vocabulary for stating it. Specify generically, dimension honestly, name the convention, and both sides stop guessing.

Frequently Asked Questions

Is “full pen” an official term on welding drawings?

No. “Full pen,” “full penetration,” and “100% pen” are shop-floor shorthand for Complete Joint Penetration (CJP). Only standardized callouts carry contractual meaning on a drawing: “CJP” in the tail, an undimensioned groove weld symbol, the melt-through symbol, the backing bar symbol, or the back weld symbol. If a drawing literally says “FULL PEN,” treat it as informal and confirm the intent with the engineer.

Which welding symbol means full penetration on a single-sided butt joint?

The melt-through symbol placed on the opposite side of the reference line from the groove weld symbol. It tells the welder to reinforce the root with weld metal on the back side, and the required reinforcement height can be dimensioned to the left of the melt-through symbol. A groove weld symbol with no depth-of-bevel or weld-size dimension also specifies CJP.

What is the difference between CJP and PJP on a symbol?

A CJP groove weld extends completely through the thickness of the components joined and should have the same strength as the components being joined. A PJP groove weld does not extend completely through, and its depth is specified by a depth-of-bevel dimension with the required weld size in parentheses to the left of the groove symbol. Adding those dimensions to a groove symbol converts a full-pen callout into a partial-penetration callout.

Should CJP be written in the tail of the welding symbol?

It is a legitimate method — arrow, reference line, and “CJP” in the tail — and it specifies the required condition without dictating how to achieve it. It works well when shop equipment or procedure is not yet known. The drawback is that it leaves procedure, filler metal, preheat, and inspection acceptance to the WPS and the applicable code, so the drawing must reference those explicitly.

Do AWS and ISO welding symbols place information on the same side of the reference line?

No, and this is a classic source of error on international work. AWS A2.4 puts arrow-side information below the reference line and other-side information above; ISO 2553 does the exact opposite. ISO also allows a fillet weld size to be given as the throat dimension while AWS always uses the leg dimension, and ISO defines pitch as clear distance between segments while AWS uses center-to-center spacing.

What is the difference between a back weld and a backing weld?

A back weld is made after the required groove weld, typically following gouging of the root, to assure full weld metal through the joint. A backing weld is a bead-type weld deposited on the opposite side of the regular weld, usually to keep the root from melting through. The memory aid that works: to do a back weld you go back to the root side after the main weld is complete.

What does the small circle at the arrow mean?

It is the weld-all-around symbol, placed at the junction of the arrow and the reference line. It requires the weld to run continuously around the entire member or joint — typical for pipe rings, tank shell seams, and hollow structural sections. It changes the length of the weld but not the penetration requirement, so it is often paired with a groove symbol or melt-through callout for full-pen work.

Is the number to the left of a fillet weld symbol the effective throat?

No. In AWS practice that number is the leg size of the fillet, and the effective throat of an equal-leg fillet is roughly 70 percent of the leg. ISO 2553 allows either the leg or the throat to be specified, so an ISO drawing and an AWS drawing with the same number can mean different weld metal. Always check which convention the drawing states.

How do you verify a full-penetration weld was actually achieved?

Start with visual inspection, then apply the surface and volumetric methods the code requires — penetrant or magnetic particle testing for surface-breaking defects, and ultrasonic or radiographic testing for internal fusion. The drawing details matter: a melt-through callout means a flat root reinforcement of a specified height must exist on the back side, and a permanent backing bar introduces a crevice that ultrasonic testing must account for. A failed CJP weld is normally repaired or excavated and rewelded, never simply accepted.