9.3 ISO 2553 Welding Symbols
Key Takeaways
- ISO 2553 welding symbols use a reference line, an arrow line, and optional tail; weld type symbols and dimensions are placed systematically relative to the reference line.
- Arrow side vs other side (and dual-reference-line System A awareness) tell the fabricator which side of the joint receives which weld—misreading this is a top exam and shop error.
- Fillet and butt (groove) symbols have distinctive shapes; dimensions for size, length, and intermittent pitch must be read in the standard’s positions.
- The tail carries complementary information such as process (ISO 4063), WPS reference, or consumable notes—not the basic weld geometry itself.
- Supplementary symbols cover all-around welds, field (site) welds, intermittent welds, and other modifiers; inspectors verify the built weld matches the symbol package.
9.3 ISO 2553 Welding Symbols
Quick Answer: ISO 2553 is the international welding-symbol language. Master the reference line, arrow side vs other side, basic fillet and butt symbols, the tail (process/WPS), and supplementary marks for intermittent, all-around, and field welds. Most exam traps are side errors and misread dimensions.
Inspectors live in drawings. If you misread a welding symbol, you accept the wrong size, wrong side, or wrong process. IWI-S candidates must read ISO 2553-style symbols fluently even when a project also uses AWS A2.4 conventions—know which system the drawing states.
Anatomy of an ISO 2553 Welding Symbol
A complete symbol set typically includes:
- Arrow line — points to the joint (or joint side) being described
- Reference line — horizontal line where weld symbols and dimensions are written
- Basic weld symbol(s) — shape identifying fillet, square butt, V butt, bevel, U, J, plug, etc.
- Dimensions — size (throat a or leg z), length, pitch for intermittent welds, etc.
- Supplementary symbols — flat/convex/concave finish, all-around, field weld, backing, spacer, etc.
- Tail (optional) — process numbers, WPS ID, filler classification, other notes
- Complementary indications — sequence numbers, staggered intermittent marks, etc. as applicable
The arrow is not decoration: it identifies the joint and, with the symbol position, which side is the arrow side.
Arrow Side vs Other Side
Core idea
- Arrow side: the side of the joint to which the arrow points
- Other side: the opposite side of the joint
In ISO 2553 practice, weld symbols are positioned on the reference line to indicate which side is welded. Candidates must learn the placement rule used on the project’s drawings (including dual-reference-line System A awareness—see below). Getting arrow side and other side reversed produces a weld on the inaccessible or wrong face—classic reject.
Practical inspector checks
- Confirm physical arrow side on the assembly matches how the welder set up
- For single-bevel or single-J symbols, confirm which member is prepared (often indicated by the arrow breaking toward the prepared member in detailed systems)
- For unsymmetric joints (T-joint, branch), wrong-side welding may be impossible to NDT as planned
Exam trap: A fillet symbol only on the other-side position does not authorise a fillet on the arrow side. Both sides require symbols (or clear dual indication) when both sides are required.
System A Awareness (ISO 2553)
ISO 2553 defines systems for arranging symbols on the reference line. System A uses a dual reference line (continuous and dashed) so that:
- Symbols on one line relate to the arrow side
- Symbols on the other line relate to the other side
Many training materials emphasise System A because it reduces ambiguity. Some drawings use System B conventions closer to simplified single-line placement. For the exam and for inspection:
- Identify which system the drawing title block or welding legend specifies
- Apply that system consistently—do not mix AWS memory rules with ISO placement unless they truly coincide on that drawing
- When in doubt on site, stop and clarify with engineering rather than guessing the side
You are not required to draft complex symbols from scratch for every edge case, but you must interpret production drawings without side inversion errors.
Basic Symbols: Fillet and Butt (Groove)
Fillet weld symbol
- Typically a right triangle attached to the reference line
- Size may be given as a (design throat) or z (leg length), depending on how the drawing annotates dimensions—read the legend
- Length of weld and intermittent pitch appear as additional numbers along the symbol system
Butt / groove weld symbols (examples)
| Symbol idea | Joint preparation intent |
|---|---|
| Square butt | Square edges |
| Single-V | V groove |
| Double-V | V both sides |
| Single bevel | Bevel on one member |
| U / J | U or J groove |
| Flare / edge / plug / slot | Special joints (recognise existence) |
Butt symbols may combine with backing symbols, permanent backing notes, or melt-through indications. Full penetration vs partial penetration is a design intent communicated by the symbol package and notes—not by assuming every V is full penetration without reading depth dimensions.
Dimensions inspectors must verify on the finished weld
- Fillet leg or throat against specified a or z (section 9.4)
- Groove weld depth of preparation and effective throat if partial penetration is specified
- Weld length and spacing for intermittent welds
- Contour (flush, convex, concave) if a finish symbol requires grinding or specific profile
The Tail — Process and WPS References
The tail of the welding symbol (open end opposite the arrow connection) carries complementary information, for example:
- ISO 4063 process numbers (111 MMA, 135 MAG, 141 TIG, 121 SAW, etc.)
- WPS or pWPS reference numbers
- Filler metal designation
- NDT or quality level notes when used that way by the project
- “Reference information” that does not fit the geometric symbol
If the tail is omitted, geometric requirements still apply, but process may be controlled only by general notes or the WPS matrix. Inspectors check that the process actually used matches the tail or the governing WPS list—welding with a different process than specified is a nonconformity even if the bead looks neat.
Intermittent Welds, All-Around, and Field Welds
Intermittent fillet welds
- Welds of specified length separated by specified gaps (pitch often end-to-end centre spacing—confirm definition on the drawing/standard)
- May be chain (aligned on opposite sides) or staggered (offset) on opposite sides of a joint
- Inspector measures segment length and spacing, not only one sample fillet size
All-around symbol
- A circle at the junction of arrow and reference line typically means weld all around the joint (for example a pipe branch or a pad)
- Does not by itself define size—size still appears with the weld symbol
- Inspectors walk the full perimeter; missing a backside segment is a common site failure
Field (site) weld symbol
- Often a flag-like mark at the arrow/reference junction indicating the weld is made on site, not in the shop
- Triggers different logistics: site WPS, welder quals for field positions, weather protection, NDT access, and sometimes different acceptance logistics
- Shop inspectors should not sign off a field-only weld as complete in the shop unless the symbol was revised
Other supplementary ideas to recognise
- Contour and finish symbols (flat/flush, convex, concave; G = grinding, etc., per legend)
- Backing and spacer symbols
- Peripheral or multiple-arrow arrangements for complex joints
Common Exam and Shop Traps
| Trap | Correct thinking |
|---|---|
| Ignoring arrow side / other side | Side is part of the requirement |
| Reading AWS placement on an ISO System A drawing | Follow the stated system |
| Assuming fillet size is always leg z | May be throat a — check legend |
| Intermittent: measuring only size, not pitch | Length and spacing are mandatory |
| Tail process ignored | Process/WPS in tail is contractual |
| All-around means “big weld” | Means full periphery, not automatic oversize |
| Field flag ignored | Location of welding is specified |
| One symbol covers both sides without dual indication | Both sides need proper symbolisation |
| Partial penetration depth skipped | Depth dimension is the design throat basis |
| Confusing weld symbol with NDT symbol | Separate symbol systems; do not mix |
Inspector Workflow with Symbols
- Read title block, welding legend, and referenced standards (ISO 2553 edition note if given).
- For each joint, decode arrow side, other side, weld type, size, length, intermittent data, and tail.
- Cross-check WPS applicability (process, joint type, position, thickness).
- At fit-up and final visual, measure what the symbol demanded.
- Record nonconformities with reference to the drawing bubble/weld ID—not vague “weld looks small.”
Exam Focus for IWI-S
WTE questions often show a simplified symbol description and ask:
- Which side is welded
- What the triangle means (fillet)
- What information belongs in the tail
- What all-around or field symbols require
- How intermittent length and pitch are interpreted
Exam tip: If two answers differ only by arrow side vs other side, re-read the symbol placement slowly—side errors are deliberate distractors.
On an ISO 2553 welding symbol, complementary information such as the ISO 4063 process number or a WPS reference is normally placed:
A welding symbol shows a fillet symbol only for the other side of a T-joint. The correct interpretation is that:
ISO 2553 System A is important for inspectors to recognise because it:
A circle at the junction of the arrow and reference line on a welding symbol most commonly indicates: