ISO 2553 System A vs System B — What Changes
ISO 2553:2019 contains two parallel systems. System A is the European dual-reference-line method. System B follows the AWS A2.4 single-line method used in the US and much of Asia. The same drawing must use only one of them, and the title block should state which.
Side-by-side comparison
The differences are structural, not cosmetic. A symbol read with the wrong system can mean the weld ends up on the wrong face of the joint.
| Feature | System A | System B |
|---|---|---|
| Reference line | Solid line plus dashed identification line | Single solid line only |
| Arrow side | Symbol on the solid line | Symbol below the line |
| Other side | Symbol on the dashed line | Symbol above the line |
| Dashed line position | May be above or below the solid line | Not used |
| Fillet size | a (throat) or z (leg), letter required | Leg length, no letter |
| Dimensions order | Size before symbol, length after | Size before symbol, length-pitch after |
| Units | mm | mm or inch, per drawing |
| Weld all around | Circle at the junction | Circle at the junction (same) |
| Field weld | Flag at the junction | Flag at the junction (same) |
The dashed line is the giveaway
If the symbol has two parallel reference lines, it is System A. If it has one, it is System B — or an AWS A2.4 drawing, which reads identically. When the dashed line is omitted in System A, the weld is symmetric on both sides of the joint and no identification line is needed.
Reading rules that do not change
Some parts of the symbol are identical in both systems, which is why partial familiarity is dangerous.
- The arrow points at the joint; the arrow side is the face the arrow touches.
- The elementary symbols (fillet triangle, V, U, J, square) are the same glyphs.
- Weld-all-around circle and field weld flag sit at the junction of arrow and reference line in both.
- The tail carries the process, procedure or standard reference in both.
Which system should you use?
Follow your customer's drawing standard. Within Europe, System A is the default and the one most inspection bodies expect. For export to the US, or for suppliers working to AWS D1.1, System B removes a translation step. Never mix systems inside one drawing set, and state the system explicitly next to the title block.
Frequently asked questions
What is the difference between ISO 2553 System A and System B?
System A uses a solid reference line plus a dashed identification line and places the other-side symbol on the dashed line. System B uses a single reference line and places the arrow-side symbol below it and the other-side symbol above it, exactly like AWS A2.4.
How do I know which system a drawing uses?
Look for the dashed identification line. Two parallel reference lines means System A; a single line means System B. The title block should also state it.
Is System B the same as AWS A2.4?
Functionally yes for reading symbols. System B was written to align ISO 2553 with the AWS A2.4 single-line convention, though AWS drawings usually give sizes in inches.
Can I mix System A and System B on one drawing?
No. One drawing set uses one system. Mixing them makes the arrow side ambiguous and is a common cause of welds placed on the wrong face.
Related pages
ISO 2553 vs AWS A2.4
Full comparison of the European and US welding symbol conventions.
Fillet weld: a-dimension vs z-dimension
Throat or leg? Conversion table, sizing rules and the mistakes that cost the most.
Intermittent welds and pitch
What 50 (100) means, chain vs staggered, and when you must not use them.
Form and position tolerances
ISO 1101 GD&T symbols explained visually, plus ISO 2768 and 13920.