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.
Reads asArrow 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
Each one is a drawing, the sentence it comes out as, and then the mark-by-mark account of why. Read the sheet first and say the sentence yourself before you look down.
Example 1: Different sizes on each face
TitleDifferent sizes on each faceRefArrow vs other sideEx1 / 2Reads as
Weld a 1/2 fillet on the face the arrow touches and a 1/4 on the far face.
Which mark says so
- fillet symbolBelow the reference lineA fillet weld on the arrow side of the joint
- 1/2Left of the arrow-side symbolLeg size of the fillet
- fillet symbolAbove the reference lineA fillet weld on the far side of the joint
- 1/4Left of the other-side symbolLeg size of the fillet
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.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
Example 2: Groove one side, back weld the other
TitleGroove one side, back weld the otherRefArrow vs other sideEx2 / 2Reads as
Prepare and fill the arrow-side groove, then back weld the opposite face.
Which mark says so
- bevel-groove symbolBelow the reference lineA bevel-groove weld on the arrow side of the joint
- 1/2Left of the arrow-side symbolGroove weld size - how deep the weld itself goes
- 45°Beyond the outer face of the arrow-side symbolIncluded angle the joint is prepared to
- 1/4Inside the arrow-side symbolRoot opening - the gap left between the members before welding
- back symbolAbove the reference lineA back weld on the far side of the joint
- Broken leaderOn the arrow itselfThe arrow points at the member to be prepared
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.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
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.