Bridle Joint

A bridle joint is a close cousin of the mortise and tenon: one piece gets a tenon, the other a matching slot, but where a true mortise is a closed cavity, a bridle joint's slot is open on both edges, straddling the full width of the piece like a saddle (hence "bridle"). It's most often used at a frame corner, where the two mating pieces are the same thickness and a fully enclosed mortise isn't practical.

Because the slot is open, a bridle joint is considerably easier to cut accurately and to trim to a snug fit after the fact than a blind mortise -- there's direct access to check and pare the fit from both sides. The tradeoff is less glue surface and a slightly weaker joint than an equivalent mortise and tenon, since there's less surrounding material holding the joint together.