Why does ISO call groove welds "butt welds"?

It is the same weld with a different name. AWS names groove welds after the groove: V-groove, bevel-groove. ISO 2553 names them after the joint and the preparation: single-V butt weld, single-bevel butt weld.

Are plug welds and slot welds the same symbol?

Yes. Both use a rectangle. A plug weld fills a round hole and a slot weld fills a long hole. The drawing or the dimensions tell you which.

What does a circle drawn across the reference line mean?

A spot weld circle centered on the line has no arrow side or other side. This is used for resistance spot welds, where the weld forms between the sheets. A circle above or below the line, such as an arc spot weld, does have a side.

Why is the upright line always on the left?

It is a drawing rule for the fillet, bevel and J symbols. The symbol keeps the same orientation whichever way the real joint faces, so you never read anything into which side the upright line is on.

Welding symbols: lesson 2 of 7

Weld types: name the symbol

Each weld type has its own small shape on the reference line. Name the weld from the shape. In ISO mode the answers use ISO 2553 names.

What weld does this symbol call for?

The eight basic weld symbols

The shape is a simple picture of the weld or the edge preparation, seen from the end. These are drawn on the arrow side here. On the other side, the same shape is drawn above the line.

AWS name	ISO 2553 name	What it means
Fillet weld	Fillet weld	Its upright side is always drawn on the left, whichever way the joint faces.
Square-groove weld	Square butt weld	The plate edges stay square, with no bevel.
V-groove weld	Single-V butt weld	Both edges are bevelled, so together they form a V.
Bevel-groove weld	Single-bevel butt weld	Only one edge gets a bevel. The upright line is always drawn on the left, and the arrow points to the part that gets the bevel.
U-groove weld	Single-U butt weld	Both edges get a curved, U-shaped preparation.
J-groove weld	Single-J butt weld	One edge gets a curved preparation and the other stays square. The upright line is always on the left, and the arrow points to the part that gets prepared.
Plug or slot weld	Plug or slot weld	Weld metal fills a round hole (plug) or a long hole (slot) in one part to join it to the part behind it.
Spot weld	Spot weld	Small round welds join overlapping sheets.

How to tell them apart fast

Triangle: fillet weld.
Straight lines only: square (two parallel lines), V (two slanted lines), bevel (one upright, one slanted).
Curves: U (stem with a cup) or J (upright line with a curve).
Box: plug or slot weld. Circle: spot weld.

For bevel and J welds, only one part gets prepared. The arrow points at that part, and AWS drawings break the arrow line when it needs to make that clear.

Questions

Why does ISO call groove welds "butt welds"?: It is the same weld with a different name. AWS names groove welds after the groove: V-groove, bevel-groove. ISO 2553 names them after the joint and the preparation: single-V butt weld, single-bevel butt weld.
Are plug welds and slot welds the same symbol?: Yes. Both use a rectangle. A plug weld fills a round hole and a slot weld fills a long hole. The drawing or the dimensions tell you which.
What does a circle drawn across the reference line mean?: A spot weld circle centered on the line has no arrow side or other side. This is used for resistance spot welds, where the weld forms between the sheets. A circle above or below the line, such as an arc spot weld, does have a side.
Why is the upright line always on the left?: It is a drawing rule for the fillet, bevel and J symbols. The symbol keeps the same orientation whichever way the real joint faces, so you never read anything into which side the upright line is on.

Explained in our own words. For the full rules, see AWS A2.4:2020 and ISO 2553:2019.