Corner joint: meaning and how to check it

Fillet Gauge editorial teamSources checked 25 Oct 20263 min read

A corner joint is two members that meet at or near their edges at an angle, usually close to 90 degrees, so the end view looks like an L. Box sections, tanks and frames are built from them. The weld can sit outside the corner, inside it, or both, depending on how the edges are placed.

Two corner joints drawn end on: an open corner where both edges meet at the outside of the L and weld fills the corner, and a closed corner where one plate sits on the other's face with a fillet inside — Open corner on the left, closed corner on the right. Both are corner joints.

How to recognise a corner joint

Look for the L. Both members end at the corner. Neither one carries on past it.
Open or closed. In an open corner, the edges meet at the outside point and leave a V-shaped space to weld into from outside. In a closed corner, one plate's edge sits on the other plate's face, flush with its edge, and the weld usually goes inside the corner as a fillet.
Half-open. Some fitters set the edges partway, leaving a small step. It's still a corner joint; the setup just changes the weld size the joint can take.

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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?

Weld toeWeld rootWeld faceDepth of fusion

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Common mix-ups

Corner versus T. Slide the upright plate in from the edge of the base plate and the L becomes a T. If the base plate carries on beyond the upright on both sides, it's a T-joint.
Corner versus edge. In an edge joint, the two members lie side by side, faces parallel, and the weld runs along their edges. In a corner joint they meet at an angle.
Joint versus weld. "Corner weld" isn't a weld type. The weld in a corner joint is a fillet, a groove weld or both, and the drawing says which.
Which plate is which. On a closed corner, the plate whose edge sits on the other's face sets where the fillet goes. Read the detail before you measure.

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Corner joints on the CWI exam

Joint types live in Definitions and Terminology, at least 12% of Part A, which you sit without books. A sketch of an L with a weld in it should read as a corner joint at a glance, whatever the weld.

On Part B, Utilization of Specifications and Drawings is at least 8% of the practical. Reading any joint detail, corners included, starts with naming the joint correctly before you look at the weld.

On a D1.1 Part C, corner joints come up under prequalified joint details in Clause 5 and in design in Clause 4. Use your own copy for the details; this page doesn't reproduce them.

Practice it

The CWI Part A flashcards include lamellar tearing, the base-metal crack that tends to turn up in T and corner joints loaded through the plate thickness. The welding terms glossary lays the five joint types side by side so you can compare an L, a T and a lap in one view.

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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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