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Plug and slot weld symbol: what it means on a drawing

Fillet Gauge editorial teamSources checked 4 min read

The plug and slot weld symbol is a rectangle on the reference line. It calls for a weld that fills a hole in one member and fuses it to the member underneath: a plug weld when the hole is round, a slot weld when the hole is elongated. Below the line, the hole is in the arrow-side member.

A lap joint drawn end on with a hole in the top plate filled with weld metal, beside a rectangle symbol marked 3/4 on the left and 4 on the right, and top views of a round plug and an elongated slot
One mark for both welds. The hole's shape tells them apart. Numbers are invented.
Use this diagram

How to read it

  1. Find the side. Below the line, the hole is in the member on the arrow side of the joint. Above the line, it's in the other member. A plug or slot weld goes through one member only, so you won't see the rectangle on both sides of the line for one joint.
  2. Plug or slot? The rectangle is shared. The dimensions and the drawing tell you which: a single diameter and round holes for plugs, a width plus a length and a slot outline for slots.
  3. Read the left side. For a plug weld, the number on the left is the size, the hole diameter at the faying surface where the two members meet. For a slot weld, it's the slot width.
  4. Read inside the rectangle. A number inside is the depth of fill. No number there means the hole is filled completely.
  5. Read the right side. For plugs, the number on the right is the pitch, center to center. For slots, the length and pitch go there, and the drawing shows which way the slots run.

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Symbols - Welding and NDE

On a welding symbol, what does a circle at the junction of the arrow and the reference line indicate?

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Common mix-ups

  • Plug weld or fillet in a hole. A fillet weld run around the inside edge of a hole is a fillet weld, drawn with the triangle and usually the all-around circle. A plug weld fills the hole with weld metal.
  • Plug or spot. Both join overlapping members at points. A plug weld needs a hole and uses the rectangle. A spot weld needs no hole and uses a circle.
  • The rectangle opposite a groove. A rectangle drawn on the far side of a groove weld symbol is not a plug weld. It marks backing material at the root of the groove.
  • Which member has the hole. It's the one on the side the symbol is on. Drilling the wrong plate is an expensive way to learn this.

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On the CWI exam

Plug and slot symbols sit in Symbols, Welding and NDE, at least 10% of CWI Part A, which is closed book. They show up less often than fillets and grooves on structural drawings, which is exactly why they're easy points to lose. Part B drawings use them under Utilization of Specifications and Drawings, at least 8% of the practical exam.

Practice it

Pick the plug weld in the welding symbol builder, move it from one side of the line to the other and read the result. Then find the rectangle among the basic shapes on the welding symbols chart, and compare it with the most common joining weld on the fillet weld symbol page.

Rectangle sorted? Check the rest of Part A

Take the free 10-question check. It's weighted to the Part A outline, scored by domain and needs no signup, so you see whether symbols or another domain costs you the most points. The Part A course gives you a lesson on reading welding and NDE symbols and on every other exam subject, two full-length timed practice exams, and a worked explanation with every answer.

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Facts checked 24 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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