A specialized recess, mortise, or slot cut into intersecting timbers to accommodate a wooden key or wedge. Once the joint is assembled, driving the key into the keyhole locks the pieces tightly together, drawing the shoulders flush without the need for glue or metal fasteners. It is a hallmark of traditional timber framing and knockdown furniture.
Heard on the job
Cut the keyhole slightly tapered so the wedge pulls the whole joint tight as you drive it home.