GaugePass

Weld symbol

Arrow side versus other side

The single fact the whole notation rests on: information below the reference line applies to the side of the joint the arrow touches, and information above it applies to the opposite side.

1/43/8
The symbol as it appears on the drawing
arrowarrow sideother side
The joint it produces, in cross-section

Reads as: Arrow side: fillet weld, 3/8 size. Other side: fillet weld, 1/4 size.

How to read it

Below the line means arrow side. Above the line means the other side. That is the entire rule, and everything else in welding-symbol notation is built on top of it. Get it backwards and you will weld the wrong face of every joint on the drawing, which is why it is worth over-learning rather than half-remembering.

The arrow side is defined by where the arrow tip physically lands, not by which direction the arrow came from or which way it points. An arrow can approach a joint from above, from below, from the left, or bent around an obstruction, and none of that matters. What matters is which face of the joint the tip is touching when it arrives.

That definition holds even when it feels counterintuitive. An arrow pointing downward at the top face of a plate makes that top face the arrow side, and information below the reference line applies to it, even though below the line and the top of the plate feel like opposites. The reference line's geometry and the part's geometry are unrelated coordinate systems.

For a handful of symbols the distinction does not apply. A surfacing weld is always on the arrow side because there is no other side to a surface, and a stud weld is likewise placed on the face the arrow touches. Resistance welds that form between sheets may be drawn centred on the line where the process itself decides access. Everything else respects the rule strictly.

Where each dimension goes

Below the reference line
Applies to the face of the joint that the arrow tip physically touches.
Above the reference line
Applies to the opposite face of the same joint.
Arrow direction
Irrelevant to the reading. Only where the tip lands defines the arrow side.
Symbols with no other side
Surfacing and stud welds sit on the arrow side only, because no opposite face exists for them.
Both sides together
Symbols above and below the line describe two operations on one joint, each dimensioned independently.

Worked examples

Different sizes on each face

1/41/2

“Weld a 1/2 fillet on the face the arrow touches and a 1/4 on the far face.”

Each triangle is read against its own side of the line. The lower symbol governs the face the arrow tip lands on and the upper governs the opposite face of the same joint.

Groove one side, back weld the other

1/245°1/4

“Prepare and fill the arrow-side groove, then back weld the opposite face.”

The two symbols sit on opposite sides of the line, so they describe operations on opposite faces of one joint rather than on two different joints in the assembly.

Common mistakes

  • Deciding the arrow side from the direction the arrow points instead of from where its tip lands.
  • Assuming below the line means the underside of the part rather than the arrow side of the joint.
  • Mirroring one side's dimensions onto the other when each side carries its own numbers.
  • Applying the rule to surfacing or stud welds, which have no other side at all.

Typical joints

Tee joints welded on one or both facesButt joints welded from one side and treated from the otherLap joints with welds on either faceCorner joints where access differs side to side

Related symbols

Reference line basicsEvery welding symbol is built on one horizontal reference line with an arrow at one end and an optional tail at the other. Where information sits on that line is what gives it meaning.Fillet weldThe right triangle on the reference line calls for a fillet weld. The number to its left is the leg size, and the side of the line it sits on tells you which face of the joint gets welded.Double filletA fillet triangle above and below the reference line means both faces of the same joint get welded. Each triangle carries its own size and length, so the two welds do not have to match.Multiple reference linesStacked reference lines on one arrow spell out a sequence of operations. The line nearest the arrow is done first, and each line further out is done after the one before it.

Practise reading welds, not just symbols

The drawing tells you what the weld should be. The gauge trainer is where you find out whether you can tell what it actually is.

Open the gauge trainer