Welding terms glossary: joints, weld parts and symbols
Welding has its own vocabulary, and the CWI exam grades it word for word. This glossary groups the terms you'll meet most on drawings, in procedures and on the shop floor, with a one-line meaning for each and a full page behind it. Every page has an original drawing, how to spot or measure the thing, and the mix-ups that cost points.
The standard behind these terms is AWS A3.0, Standard Welding Terms and Definitions, and the symbol pages follow AWS A2.4. Every definition and drawing here is in our own words; the standards' wording and figures stay in the standards. On structural work, D1.1 Clause 3 lists the terms that code relies on, and you read it in your own copy.
Joint types
How the members meet, before anyone picks a weld.
| Term | In one line | Full page |
|---|---|---|
| Butt joint | Two members edge to edge in about the same plane | Butt joint |
| T-joint | One member's edge against the other's face at about 90 degrees | T-joint |
| Corner joint | Two members meeting at their edges in an L, open or closed | Corner joint |
| Lap joint | Two members overlapping in parallel planes | Lap joint |
| Edge joint | Members face to face, welded along their flush edges | Edge joint |
The joint type never changes with the weld. A butt joint can take a square, V, bevel, U or J groove, and a T-joint can take fillets, a groove weld or both. Name the joint first, then read the weld.
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Definitions and Terminology
A fatigue crack in a cyclically loaded fillet-welded attachment started along the line where the surface of the weld blends into the flange plate. At which weld feature did the crack start?
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Parts of a weld
Where to look on a finished weld, and what each surface or line is called.
| Term | In one line | Full page |
|---|---|---|
| Weld face | The weld's outer surface on the welder's side, toe to toe | Weld face |
| Weld root | The bottom of the weld metal: far-side edges on a groove, inside corner on a fillet | Weld root |
| Weld toe | The line where the face meets the base metal | Weld toe |
| Weld reinforcement | Groove weld metal standing past the plate surface, face or root side | Weld reinforcement |
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Fillet weld size and throats
The leg is what you measure; the throats are what the strength depends on. Learn all four together, because the differences between them are the point.
| Term | In one line | Full page |
|---|---|---|
| Fillet weld leg | Root of the joint to the toe, along one member | Fillet weld leg |
| Theoretical throat | Root to the long side of the largest right triangle that fits | Theoretical throat |
| Effective throat | Weld root to a straight toe-to-toe face, convexity left out | Effective throat |
| Actual throat | Weld root to the real face, convexity included | Actual throat |
On an equal-leg fillet with a flat face and no fusion past the root, the theoretical and effective throats are the same, about 0.707 times the leg. Convexity only ever adds to the actual throat.
Joint preparation and fit-up
What gets checked before the first pass, while it can still be fixed.
| Term | In one line | Full page |
|---|---|---|
| Root opening | The gap between members at the root | Root opening |
| Root face (land) | The flat strip left at the bottom of a bevel | Root face |
| Bevel angle and groove angle | One member's cut versus the whole opening | Bevel angle vs groove angle |
| Backing | Material under the root that holds the molten metal in | Weld backing |
Heat and passes
How the weld is built up and what the heat does to the metal around it.
| Term | In one line | Full page |
|---|---|---|
| Heat-affected zone | Base metal changed by heat but never melted | Heat-affected zone |
| Weld pass and weld layer | One trip along the joint versus one level of the weld | Weld pass vs weld layer |
| Interpass temperature | Joint temperature just before the next pass starts | Interpass temperature |
Welding symbols
Each symbol has its own page with a drawing of the symbol beside the joint it describes. The welding symbols chart puts them all on one printable page.
| Symbol | In one line | Full page |
|---|---|---|
| Fillet | A triangle: a fillet weld, size on the left | Fillet weld symbol |
| Weld all around | A circle at the arrow bend | Weld all around symbol |
| Field weld | A flag at the arrow bend: made at the job site | Field weld symbol |
| Stitch | Length and pitch after the symbol | Stitch weld symbol |
| Square groove | Two strokes: square edges | Square groove weld symbol |
| V-groove | Both members beveled | V-groove weld symbol |
| Double-V | A V on each side of the line | Double-V groove weld symbol |
| Bevel groove | One member beveled; the arrow break picks which | Bevel groove weld symbol |
| U-groove | Both members get a curved prep | U-groove weld symbol |
| J-groove | One member gets a curved prep | J-groove weld symbol |
| Flare-V | Two rounded members side by side | Flare-V groove weld symbol |
| Flare-bevel | A rounded member against a flat one | Flare-bevel groove weld symbol |
| Plug or slot | A weld filling a hole in one member | Plug and slot weld symbol |
| Spot | Welds at points between overlapping members | Spot weld symbol |
| Seam | A continuous weld along an overlap | Seam weld symbol |
| Back or backing weld | An open half-circle opposite the groove | Back weld and backing weld symbol |
| Melt-through | A filled half-circle: root reinforcement from one side | Melt-through symbol |
| Edge | Three joined strokes on members laid together | Edge weld symbol |
| Surfacing | Two arcs: weld metal built up on a surface | Surfacing weld symbol |
| Contour | Flat, convex or concave face, plus a finish letter | Weld contour symbol |
Weld discontinuities
What each flaw looks like, how it forms and how to tell it from its look-alike. The weld discontinuities guide groups them by where they sit in the weld.
- Weld porosity, Slag inclusion, Tungsten inclusion, Incomplete fusion, Incomplete joint penetration, Weld undercut, Weld overlap, Weld underfill, Longitudinal weld crack, Transverse weld crack.
- Crater crack, Toe crack, Root crack, Underbead crack, Lamination in steel plate, Lamellar tearing, Arc strike, Weld spatter, Excessive weld reinforcement, Burn-through.
How these terms show up on the CWI exam
Definitions and Terminology is at least 12% of Part A, and Symbols, Welding and NDE at least 10%. Part A is closed book, so every word on this page has to be in your head, not in a binder.
Part B uses the same words with tools in your hand. Legs, convexity, reinforcement and undercut at the toe are read from the replicas, and Welding Inspection and Discontinuities is at least 34% of the practical.
On a D1.1 Part C, the terms point you to the right clause: joint details in Clause 5, fit-up and workmanship in Clause 7, and inspection in Clause 8.
Practice with the tools
- Fillet weld calculator: legs in, throat, area and weld metal per foot out.
- Weld gauge reading trainer: fillet gauge and V-WAC style readings on generated cases.
- CWI Part A flashcards: original cards on symbols, metallurgy, discontinuities and the rest of Part A.
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Sources
Show the list
- AWS bookstore, terminology and symbols listing, American Welding Society. Retrieved 20 Oct 2026.
- CWI Part A Fundamentals Examination Information, American Welding Society, Rev. 03.01.2024. Retrieved 20 Oct 2026.
- AWS B5.1:2025, Specification for the Qualification of Welding Inspectors, American Welding Society, Tables 5.1 and 8.1. Retrieved 20 Oct 2026.
Facts checked 25 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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