AWS A2.4:2020 · Engineering Drawings

Welding Symbols on Blueprints — How to Read AWS A2.4

A complete गाइड for reading वेल्डिंग प्रतीक on engineering blueprints. Reference line, arrow, वेल्ड प्रतीक, dimensions, supplementary symbols, compound symbols, and ISO 2553 differences. Built for fabricators, inspectors, and CWI candidates.

Welding symbols on blueprints are the standardized notation per AWS A2.4:2020 that tells fabricators exactly what type of वेल्ड to make, where on the joint to make it, and how large it must be. Every welding symbol consists of a horizontal reference line, an arrow pointing to the joint, a weld symbol on the reference line, and an optional tail with process or विनिर्देश references.

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Welding Symbol — Anatomy

Four essential elements per AWS A2.4:2020

Overview

What Welding Symbols on Blueprints Communicate

Welding symbols on engineering blueprints are the standardized graphical notation defined in AWS A2.4:2020 — the latest edition of the American Welding Society मानक for symbols, brazing, and nondestructive examination. Every weld on a structural drawing is specified through this notation. The welding symbol communicates four pieces of information simultaneously: the type of weld (fillet, groove, plug, slot, seam, surfacing), the location on the joint (arrow side, other side, both sides), the आकार and length, and any process or specification आवश्यकताएँ.

Reading welding symbols on blueprints is a core competency for welders, fabricators, welding engineers, structural detailers, and Certified Welding Inspectors. The symbols eliminate ambiguity on the shop floor — the symbol tells the welder exactly what type of weld to make, where to make it, and how large it must be. A welding symbol that is misread costs significantly more to correct than the original weld, particularly on structural steel where rework involves grinding, re-preparation, and re-निरीक्षण.

The AWS A2.4 standard is referenced by major structural welding codes including AWS D1.1:2025 for structural steel, AWS D1.5 for bridges, AWS D1.6 for stainless steel, and indirectly by ASME Section IX for pressure vessel welding. North American projects almost universally use AWS A2.4. International projects more often use ISO 2553, which has subtle but critical differences समझाया later in this guide.

Per AWS A2.4:2020 Section 5: “The welding symbol is composed of an arrow line, a reference line, the weld symbol, dimensions, supplementary symbols, and a tail in which references may be placed.”

— AWS A2.4:2020, Standard Symbols for Welding, Brazing, and Nondestructive Examination

Anatomy

The Four Essential Elements of a Welding Symbol

Per AWS A2.4:2020, every welding symbol consists of four essential elements. Understanding each element in isolation is the foundation for reading any welding symbol on a blueprint, no matter how complex.

Reference Line

The reference line is the horizontal backbone of every welding symbol. It is always drawn as a solid horizontal line. The reference line is what divides the symbol into the arrow-side zone (below the line) and the other-side zone (above the line). All other elements — arrow, weld symbol, dimensions, supplementary symbols — attach to the reference line. Without a reference line, you do not have a welding symbol; you have a drawing annotation.

Arrow

The arrow extends from one end of the reference line to the joint that requires welding. The arrow can be drawn at any angle — straight, broken, or curved — but it always terminates at the joint. The arrow's direction defines the arrow side of the joint per AWS A2.4 §6.1. For most welds the arrow direction is unambiguous, but for groove welds requiring a specific bevel orientation, the arrow may be intentionally broken with a kink to indicate which member receives the bevel preparation per §6.3.

Weld Symbol

The weld symbol is the graphic placed on the reference line that indicates the type of weld required. The eight most common वेल्डिंग प्रतीक on structural drawings are: fillet (right triangle, perpendicular leg always on the left), V-groove (V), bevel-groove (single sloped line, perpendicular leg on the left), plug (rectangle), slot (elongated hexagon), spot (hollow circle), seam (hollow circle with horizontal line through it), and surfacing (parallel curved arcs). AWS A2.4:2020 recognizes 18 distinct weld symbols in total — covering specialty welds like flare-V, flare-bevel, edge, stud, and back/backing welds.

Tail

The tail is an optional bracket-shaped element at the opposite end of the reference line from the arrow. It contains process designations (such as SMAW, GMAW, FCAW, SAW), specification references (like AWS D1.1), or WPS numbers. When no tail information is needed, the tail is simply omitted — this is the most common state on simple symbols. When the tail is present, the order and meaning of items inside the tail follows the project standard or the engineer's note.

Field note: The AWS A2.4 standard treats every welding symbol as a complete instruction. If any of the four elements is missing, the welder should request clarification rather than assume. A symbol without dimensions defaults to project standards or engineer notes — do not improvise weld size in the field.

— CWI field observation, structural fabrication shop, 2026

Placement

Arrow Side vs Other Side on Blueprints

The single most-tested concept on the CWI Part B practical exam is arrow-side vs other-side placement. Per AWS A2.4:2020 §6.1, the rule is unambiguous and depends entirely on position relative to the reference line, not on दृश्य orientation of the arrow.

When the weld symbol is placed below the reference line, the weld is to be made on the arrow side of the joint — the same side that the arrow physically points to. When the weld symbol is placed above the reference line, the weld is to be made on the other side of the joint — the side opposite from where the arrow points. When weld symbols appear on both sides of the reference line — one above and one below — the welder must weld both sides of the joint with the size and type specified for each side.

This convention is consistent regardless of arrow direction, joint orientation, or page rotation. Whether the arrow points down, up, left, right, or at a 45-degree angle, the rule is the same: position relative to the reference line determines the side. This rule is the source of approximately 30 to 40 percent of CWI Part B test errors per AWS QC1 exam analysis — not because the rule is complex, but because candidates conflate visual orientation with the position rule.

Arrow Side vs Other Side — Comparison

Arrow side weld

Other side weld

Both sides — weld each face

Important caution for international projects: ISO 2553 System A uses an inverted convention for indicating other-side placement — a dashed identification line is added below or above the reference line to indicate the other side, regardless of weld symbol position. This is fundamentally different from AWS A2.4. On any drawing referencing ISO 2553, do not apply the AWS A2.4 position rule. See the dedicated ISO 2553 vs AWS A2.4 comparison.

Dimensions

Reading Dimensions on a Welding Symbol

Per AWS A2.4:2020 §8, dimensions on a welding symbol follow strict positional conventions. Misreading dimensions is the second-most-common blueprint reading error after arrow-side confusion.

Size (to the left of the symbol)

Weld size is always placed to the left of the weld symbol per §8.2.1. For fillet welds, the size is the leg length — for example, a 5/16 to the left of a fillet triangle means a 5/16-inch leg fillet. For groove welds, the size is the depth of preparation when shown without parentheses, or the effective throat when shown in parentheses per §8.2.2. Per D1.1:2025 Table 7.7, न्यूनतम फ़िलेट वेल्ड sizes depend on आधार धातु मोटाई: 1/8 inch minimum for plate up to 1/4 inch, 3/16 inch for over 1/4 to 1/2 inch, 1/4 inch for over 1/2 to 3/4 inch, and 5/16 inch for thicker than 3/4 inch.

Length (to the right of the symbol)

Weld length is placed to the right of the weld symbol per §8.3.1. A single number to the right means continuous weld of that length — for example, 8 to the right of a fillet symbol means an 8-inch continuous fillet weld. If no length is shown and there is no length-pitch notation, the weld is continuous along the entire joint.

Length-Pitch (intermittent welds)

For intermittent fillet welds, two numbers separated by a hyphen appear to the right of the symbol per §8.4. The format is length-pitch — for example, 3-12 means 3-inch weld segments with 12-inch center-to-center pitch. The unwelded gap between segments is pitch minus length: 12 minus 3 equals 9 inches of unwelded space. Critical: the pitch is center-to-center, not the gap between welds — this is the most common misread on the CWI exam. Per D1.1:2025 §4.13.2.1, अधिकतम pitch is limited to 24 times the thinner plate thickness, not to exceed 12 inches for built-up plate members.

Chain vs Staggered Intermittent

When intermittent welds are required on both sides of a joint, the symbol indicates whether they are aligned (chain, per §8.4.3) or offset (staggered, per §8.4.4). Chain intermittent welds have aligned segments above and below the reference line. Staggered welds have segments offset by half a pitch, providing more uniform stress distribution at the cost of slightly more handling complexity. Staggered intermittent is often preferred on cyclically-लोड हो गया structures per fatigue design considerations in D1.1:2025 Clause 4.6.

Supplementary Symbols

Supplementary Symbols (Field Weld, Weld-All-Around, Contour)

AWS A2.4:2020 defines several supplementary symbols that attach to the basic welding symbol to provide additional information. The three most common on structural blueprints are the weld-all-around circle, the field weld flag, and contour finishing symbols.

Weld-All-Around Circle

A hollow circle placed at the intersection of the reference line and the arrow is the weld-all-around symbol per AWS A2.4:2020 §6.11. It instructs the welder to make the specified weld continuously around the entire perimeter of the joint, regardless of joint geometry. This is most common on tube-to-plate connections, attached studs, structural pipe-to-flange joints, and any closed-loop joint. Do not confuse the weld-all-around circle (at the arrow-reference intersection) with the spot weld symbol (a hollow circle placed on the reference line). The visual similarity is the source of frequent CWI exam errors.

Field Weld Flag

A small triangular flag attached to the intersection of the reference line and the arrow is the field weld symbol per AWS A2.4:2020 §6.9. The flag points toward the tail. It instructs the welder that the weld is to be made in the field rather than in the fabrication shop. Field welds are common on structural steel erection, pipeline tie-ins, and any weld that cannot be made before transport due to size or assembly sequence. Field conditions affect process selection (कम हाइड्रोजन consumables typically required), पूर्वतापन application, weather protection, and inspection scheduling per D1.1:2025 Clause 6.

Contour Symbols

A line placed against the open side of the weld symbol indicates the desired weld face contour per AWS A2.4:2020 §7.6 for groove welds and §8.6 for fillet welds. A solid straight line means flat (or flush, depending on context). A curved line above the weld symbol means convex; a curved line below means concave. A letter following the contour symbol indicates the finishing method: G for grind, M for machine, C for chip, R for roll, or U for unspecified. The contour and finish symbols together specify both the desired final shape and how to achieve it.

Compound Symbols

Compound Welding Symbols (Multiple Welds on One Symbol)

For joints requiring multiple weld passes or weld प्रकार, AWS A2.4:2020 allows compound symbols where multiple weld symbols stack on the same reference line per §6.5. The most common compound is a ग्रूव वेल्ड with a backing weld. The groove symbol appears on the reference line in the standard position, and the backing weld symbol (a rectangle) appears on the opposite side of the reference line from the groove symbol.

Compound symbols are read in welding sequence: the symbol nearest the arrow side is the weld made first; the symbol on the other side is made after. For a CJP groove weld with a backing weld on the back side, the welder lays the backing weld first to provide root support, then completes the groove weld from the arrow side. The dimensions for each weld type are read independently — size to the left of each symbol, length to the right.

Multiple-reference-line symbols are an extension of compound symbols where a stack of two or more reference lines appears in the welding symbol. The bottom reference line specifies the first weld operation; subsequent reference lines specify follow-on operations in sequence per §6.7. This notation is most common on PJP/CJP combination joints and on welds that require sequenced backgouging operations. See the dedicated multiple reference lines page for detailed examples.

International

AWS A2.4 vs ISO 2553 on International Blueprints

AWS A2.4:2020 is the dominant standard for welding symbols on North American blueprints. ISO 2553 is the international equivalent, used on European, Japanese, Australian, and most non-North American projects. The two standards are largely compatible — most weld symbols (fillet, V-groove, plug) are visually identical — but they diverge in three significant ways that affect blueprint reading.

First, arrow-side convention. AWS A2.4 uses position relative to a single solid reference line: below the line means arrow side, above means other side. ISO 2553 System A adds a dashed identification line where the dashed line indicates other side regardless of weld symbol position. ISO 2553 System B uses a single solid reference line like AWS but may flip the arrow-side rule depending on regional convention. Always सत्यापित करें which standard and which system is in effect.

Second, dimension placement for groove welds. AWS A2.4 places groove depth to the left without parentheses (PJP only) and weld size to the left with parentheses. ISO 2553 may use slightly different conventions for groove dimensions, particularly for combined groove and surfacing welds.

Third, supplementary symbol placement. The weld-all-around circle, field weld flag, and contour symbols are similar but not identical between the two standards. ISO 2553 includes additional supplementary symbols not present in AWS A2.4, particularly for non-destructive examination requirements integrated into the welding symbol.

On international projects, the title block or general notes will reference the applicable standard. If both standards are referenced (a multinational fabrication contract), the engineer should specify which standard governs in case of conflict. Misinterpreting the standard on a drawing leads to welds on the wrong side of the joint — one of the most expensive corrective actions in fabrication.

CWI Exam Tips

Common Blueprint Reading Mistakes

The following five blueprint reading errors are the highest-frequency mistakes on the CWI Part B practical exam and on shop floor non-conformance reports per AWS QC1 candidate analysis and structural fabrication QA records.

1. Confusing Arrow Side with Other Side

The position rule is absolute: position relative to the reference line determines the side, regardless of arrow direction or page orientation. When the arrow points downward and the weld symbol is below the reference line, the weld goes on the side the arrow points to (arrow side). Do not let visual orientation override the rule.

2. Misreading Intermittent Weld Pitch

The pitch in length-pitch notation is center-to-center, not the gap between welds. For 3-12, the unwelded gap is 9 inches (12 minus 3), not 12 inches. This single misread can परिणाम in welds being placed too far apart for fatigue design.

3. Missing the Weld-All-Around Circle

A small circle at the arrow-reference intersection is easy to miss on a busy blueprint. Stopping the weld at one face on a tube-to-plate joint when the circle is present is a structural failure mode — the joint is not a closed loop, water ingress and fatigue cracking become initiation points.

4. Ignoring the Field Weld Flag

A field weld pre-fabricated in the shop can cause assembly issues at the construction site — the joint may not align after thermal distortion. Field weld flags exist for sequencing reasons; ignoring them defeats the engineer's intent.

5. Confusing AWS A2.4 with ISO 2553

On international projects, the dashed identification line in ISO 2553 System A is the most common cross-standard error. North American welders reading European drawings sometimes ignore the dashed line and apply the AWS position rule, ending up with welds on the wrong side of the joint.

Frequently Asked

Welding Symbols on Blueprints — FAQ

What does a welding symbol on a blueprint mean?

A welding symbol on a blueprint is the standardized graphical notation per AWS A2.4:2020 that tells the fabricator exactly what type of weld to make, where on the joint to make it, and what size and length the weld must be. The full welding symbol assembly consists of a horizontal reference line, an arrow pointing from the reference line to the joint, a weld symbol graphic placed on the reference line that indicates the weld type, and an optional tail containing process designations or specification references. The position of the weld symbol relative to the reference line determines arrow-side or other-side placement. Supplementary symbols add details like the weld-all-around circle, the field weld flag, and contour finishing requirements. The welding symbol is the single most important communication channel between the engineer and the welder — every weld on a structural drawing is specified via this notation.

How do you read a welding symbol on a blueprint?

Read a welding symbol on a blueprint in four steps. First, follow the arrow from the reference line to the joint to identify which joint is being specified. Second, look at the weld symbol on the reference line to identify the weld type — for example, a right triangle indicates a fillet weld per AWS A2.4 §6.3, while a V indicates a V-groove weld. Third, check whether the symbol is below the reference line (arrow side), above the reference line (other side), or on both sides (weld both faces). Fourth, read any dimensions to the left of the symbol for size, and to the right for length and pitch on intermittent welds. Then check the tail for process codes, specification references, or WPS numbers. Always confirm which standard the drawing references — AWS A2.4 and ISO 2553 use opposite arrow-side conventions in some configurations.

What are the four parts of a welding symbol on a blueprint?

The four essential elements of every welding symbol per AWS A2.4:2020 are the reference line, the arrow, the weld symbol, and the tail. The reference line is the horizontal backbone of the symbol — every other element attaches to it. The arrow extends from one end of the reference line to the joint being welded; the arrow direction defines the arrow side of the joint. The weld symbol is the graphic placed on the reference line that indicates the type of weld required — examples include the right triangle for fillet welds, the V for V-groove welds, the rectangle for plug welds, and the circle for spot welds. The tail is an optional bracket-shaped element at the opposite end of the arrow that contains process codes (such as SMAW, GMAW, FCAW), specification references, or WPS numbers. When no tail information is needed, the tail is simply omitted. Dimensions, supplementary symbols, and the all-around circle are added to these four base elements as needed.

What does the arrow side mean on a blueprint?

The arrow side is the side of the joint that the arrow physically points to on the blueprint. Per AWS A2.4:2020 §6.1, when the weld symbol is placed below the horizontal reference line, the weld is to be made on the arrow side of the joint. When the weld symbol is placed above the reference line, the weld is to be made on the other side — the side opposite from where the arrow points. When weld symbols appear on both sides of the reference line, the welder must weld both sides of the joint with the size and type specified for each side. This convention is the single most common source of CWI exam errors and shop floor confusion. Important caution: ISO 2553 System A uses an inverted convention where a dashed identification line indicates the other side rather than position above or below — always verify which standard the drawing references before interpreting placement.

What does it mean when the symbol is on both sides of the reference line?

When weld symbols appear on both sides of the reference line — one above and one below — the welder must make a weld on both sides of the joint per AWS A2.4 §6.1. The size and type of weld on each side is specified independently. For example, a 1/4-inch fillet symbol below the reference line and a 5/16-inch fillet symbol above the reference line means: weld a 1/4-inch fillet on the arrow side of the joint and a 5/16-inch fillet on the other side. If both symbols are identical, the same weld is required on both faces. Both-sided fillet welds are common on T-joints in structural steel where the joint is loaded from both faces. The dimensioning rule applies to each symbol independently — leg sizes, lengths, and pitch on intermittent welds are read from the symbol on each side.

What does the circle mean on a welding symbol?

A circle at the intersection of the reference line and the arrow is the weld-all-around supplementary symbol per AWS A2.4:2020 §6.11. It instructs the welder to make the specified weld continuously around the entire perimeter of the joint, regardless of joint geometry. The weld-all-around circle is most common on tube-to-plate connections, attached studs, structural pipe-to-flange joints, and any closed-loop joint where a continuous weld is required for strength or sealing. It eliminates the ambiguity of where to start and stop the weld. Do not confuse the weld-all-around circle (a hollow circle at the arrow-reference intersection) with the spot weld symbol (a hollow circle placed on the reference line) — they look similar but mean entirely different things. The spot weld symbol size dimension goes inside or to the left of the circle; the weld-all-around symbol has no dimension.

What does the flag mean on a welding symbol?

The flag is the field weld supplementary symbol per AWS A2.4:2020 §6.9. It is a small filled or unfilled triangular flag attached to the intersection of the reference line and the arrow, with the flag pointing toward the tail. The flag instructs the welder that this weld is to be made in the field — meaning at the construction site rather than in the fabrication shop. Field welds are common on structural steel erection, pipeline tie-ins, and any weld that cannot be made before transport due to size or assembly sequence. The distinction matters because field conditions affect process selection (low-hydrogen consumables are typically required), preheat application, weather protection, and inspection scheduling. Drawings will often combine the field weld flag with a weld-all-around circle for tube-to-base-plate connections at the field. The field weld flag does not change the weld geometry — it changes only where and under what conditions the weld is made.

How do you read a fillet weld symbol on a blueprint?

To read a fillet weld symbol on a blueprint per AWS A2.4:2020 §6.3 and §8.2.1: identify the right triangle on the reference line — this is the fillet weld symbol. The perpendicular leg of the triangle is always drawn on the left side regardless of joint orientation, which is a fixed convention not an indicator of geometry. The number to the left of the triangle is the leg size in inches or millimeters. For example, the notation 5/16 with a fillet symbol means a 5/16-inch leg fillet weld. If the symbol is below the reference line, the weld is on the arrow side; above means other side; both means weld both faces. Numbers to the right of the symbol in length-pitch format like 3-12 indicate intermittent welds — 3-inch weld segments with 12-inch center-to-center pitch. The pitch is center-to-center, not gap distance — this is the most common misread on intermittent welds. Per D1.1:2025 Table 7.7, minimum fillet weld size depends on base metal thickness.

What is the difference between AWS A2.4 and ISO 2553 welding symbols on blueprints?

AWS A2.4:2020 is the North American standard for welding symbols; ISO 2553 is the international standard used in Europe, Japan, Australia, and most non-North American projects. The two standards diverge in three significant ways. First, arrow-side convention — AWS A2.4 uses position relative to a single solid reference line (below = arrow side, above = other side), while ISO 2553 System A adds a dashed identification line where the dashed line indicates other side regardless of position. Second, weld symbol shape — most weld symbols (fillet, V-groove, plug) are visually identical between the two standards, but a few have orientation differences. Third, dimension placement — both standards put leg sizes to the left of fillet symbols, but ISO 2553 may use different conventions for groove dimensions and supplementary symbols. On international projects, always verify which standard the drawing references — it is typically noted in the title block or general notes. Misinterpreting the standard on a drawing leads to welds on the wrong side of the joint, which is one of the most expensive corrective actions in fabrication.

What are common mistakes when reading welding symbols on blueprints?

The five most common blueprint reading mistakes per CWI exam analysis and shop floor experience are: First, confusing arrow side with other side — particularly when the arrow points downward or at an angle that makes orientation ambiguous; the rule is always position relative to the reference line, not visual orientation. Second, misreading intermittent weld pitch as the gap between segments rather than center-to-center distance — for 3-12 notation, the unwelded gap is 9 inches (12 minus 3), not 12. Third, missing the all-around circle and stopping the weld at one face only on a tube-to-plate joint. Fourth, ignoring the field weld flag and pre-fabricating welds that should be made in the field, leading to assembly issues. Fifth, confusing AWS A2.4 and ISO 2553 conventions on international projects, particularly the dashed identification line in ISO 2553 System A. CWI exam preparation should drill arrow-side determination, intermittent weld pitch calculation, and supplementary symbol identification — these are the highest-frequency exam errors.

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