The Symbol on the Line
An AWS weld symbol compresses a joint's full instructions — weld type, size, side, and site — onto one reference line with an arrow. · 11 min
A fabrication drawing has no room for paragraphs. Instead, an arrow touches the joint and a short horizontal line — the reference line — carries everything you need to know: what weld, what size, which side, made where. Welders call the whole mark a weld symbol, and it is a standardized language (AWS A2.4 in North America): the same mark reads the same in any shop. This folio teaches you to read it aloud as a plain work order.
Guess before you learn
A fillet triangle sits below the reference line. Where does the weld go?
Below the line means arrow side — weld where the arrow points. Above the line means other side. Most people guess it the other way around at first; the convention feels upside-down until you have read a dozen of them. By the end of this folio it will be automatic.
9–12
3–5
The arrow points at the joint. The line attached to the arrow carries small marks, and each mark is an instruction. A triangle means a fillet weld. A number beside the triangle tells you how long to make its legs.
One rule does most of the work: a mark below the line means weld the side the arrow touches; a mark above the line means weld the opposite side. The drawing can order both at once by marking both.
6–8
The reference line is always horizontal; the arrow runs from one end down to the joint. Marks below the line command the arrow side; marks above command the other side; marks on both command both sides. For a fillet symbol, the number to its left is the leg size, in the drawing's units.
Two more marks finish the basics: a small circle where the arrow meets the line means weld all around the joint, and a flag there means a field weld — made at the site, not in the shop. A forked tail at the far end holds written notes, usually the process: SMAW, GMAW.
9–12
Groove welds carry their preparation inside the symbol: a V mark with the groove angle written in it and the root opening beneath. Fillet symbols can also order intermittent welds — a length and pitch after the size, such as 50–150: welds 50 mm long, spaced 150 mm centre to centre.
The arrow itself can carry meaning: for bevel-groove joints it kinks toward the member to be beveled. And placement is absolute — below the line means arrow side even when the arrow points up, left, or around a corner of the drawing. The reference line never rotates; only the joint does.
K–2
A drawing is a picture of what to build. An arrow points at a corner. A small triangle sits on a line near the arrow. It means: put a weld in this corner.
If the triangle hangs below the line, weld the side the arrow touches. If it sits on top, weld the other side.
Undergrad
The weld symbol is contract language: on a stamped drawing it carries the same force as a dimension. The convention exists to remove ambiguity — a prose note like weld near side fails the moment a view is mirrored or flipped, while arrow-side and other-side are defined relative to the arrow, which survives any projection.
Reading discipline is fixed: identify the joint, resolve the sides, then read outward — weld type, size, length and pitch, contour and finish marks, then the tail. Anything the symbol leaves unsaid is governed by the welding procedure specification, never by the welder's preference.
Postgrad
Two standards families coexist: AWS A2.4 and ISO 2553. Older ISO editions marked the other side with a dashed identification line — a mirror-image trap for anyone moving between American and European drawings. Misread sides remain among the most common detailing nonconformances found in fabrication audits.
The symbol also interlocks with qualification: a field-weld flag can silently change the required procedure — wind limits push field work toward SMAW or self-shielded flux-cored over gas-shielded processes — and a specified contour invokes finishing operations with their own inspection points. A symbol is only fully read against its governing code and WPS.
reference line
The horizontal line the weld marks ride on. Below it: weld the arrow side. Above it: weld the other side.
Now put it to work. A symbol is read in a fixed order, the same every time: find the joint the arrow touches; check which side of the line the mark sits on; name the weld from the mark's shape; read the size to its left; check the bend of the arrow for a circle or a flag; finish with the tail. Read in that order, every callout collapses into one plain sentence — a work order you could hand to another welder without the drawing.
Try a full translation. A drawing shows a tee joint. The arrow touches the left side of the web. A triangle hangs below the line with a 6 to its left; a flag stands at the bend; the tail reads SMAW. Before the stepper below reveals it, say the work order out loud.
Translate the callout into a work order — the steps fade as you master them
Arrow touches the web's left side; triangle below the line — weld the arrow side
Fillet weld, 6 mm legs
Flag: weld it in the field. Tail: use SMAW
Run a 6 mm fillet on the arrow side of the web, on site, with stick
You can now read the most common callouts in any fabrication shop — fillets sized, sided, and sited. Groove symbols carry more furniture (angles, root openings) but obey the same line and the same sides. Next folio the drawing starts fighting back: the metal moves when you weld it.
Practice — new ink and old, interleaved
1.Recite the six-step reading order from memory.
Arrow to the joint; side of the line; weld shape; size; circle or flag at the bend; tail note.
How close were you? Grade yourself honestly — it sets your review date.
2.A square tube post welded upright onto a flat baseplate?
3.Put the first three steps of reading a callout in order.
- Find the joint the arrow touches
- Check which side of the line the mark is on
- Name the weld from the mark's shape
4.The five joints, from memory, each with its defining geometry.
Butt — edge to edge in one plane; lap — faces overlapping; tee — edge into face at 90 degrees; corner — edges meeting in an L; edge — faces parallel with edges together.
How close were you? Grade yourself honestly — it sets your review date.
5.The mark is a V shape open to the top, sitting above the line — not a triangle. What is being ordered, and where?
6.Inch drawing: 1/4 sits left of the fillet triangle. What leg size, in inches?
7.In one sentence: what two protections does the burning flux give the molten weld?
8.A corner joint shows a fillet triangle below the line but no size number anywhere near it. What do you do?
9.In E7018, the 70 means: