In welding, the common corner joints are classified as flush or closed, half open, and full open.

The corner joint is used to join two members located at approximately right angles to each other in the form of an L. The fillet weld corner joint (view A, fig. 6-19) is used in the construction of boxes, box frames, tanks, and similar fabrications.

Welding  corner joints for sheets and plate.

The closed corner joint (view B, fig. 6-19) is used on light sheet metal, usually 20 gage or less, and on lighter sheets when high strength is not required at the joint. In making the joint by oxyacetylene welding, the overlapping edge is melted down, and little or no filler metal is added. In arc welding, only a very light bead is required to make the joint. When the closed joint is used for heavy sections, the lapped plate is V beveled or U grooved to permit penetration to root of the joint.

Half open comer joints are suitable for material 12 gage and heavier. This joint is used when welding can only be performed on one side and when loads will not be severe.

The open corner joint (view C, fig. 6-19) is used on heavier sheets and plates. The two edges are melted down and filler metal is added to fill up the corner. This type of joint is the strongest of the corner joints.

f. Corner joints on heavy plates are welded from both sides as shown in view D, figure 6-19. The joint is first welded from the outside, then reinforced from the back side with a seal bead.