keyhole

A keyhole in carpentry is a rectangular or trapezoidal cavity cut into the face or edge of a beam, timber joint, or timber-framed structure. It receives a wooden key, also called a key wedge or through tenon, which locks two beams together by friction and mechanical interlock. The key is driven into the keyhole perpendicular to the grain of the wood, compressing the fibers of the receiving timber and creating a joint that resists separation and shear forces.

The keyhole opening is typically larger at its entry point than in its depth, cut at a slight angle or with beveled walls. This taper allows the key to wedge tightly as it is driven home. The depth of the keyhole is calculated so that the key fully engages without breaking through the opposite face of the beam. In timber-framed buildings and large carpentry work, keyholes are commonly found in beam connections, collar ties, and lap joints where bolts or metal fasteners are impractical or undesirable.

Variants and materials

A keyhole may be cut to receive a hardwood key, softwood key, or in some cases a composite key. Oak and ash keys are preferred for their hardness and resistance to crushing. The keyhole itself is usually cut with a chisel and saw or with a router jig. Industrial production uses mortising machines. The size varies widely: small keyholes for roof trusses might be 25 by 50 millimeters, while large structural keyholes in girders can exceed 75 by 150 millimeters.

Keyholes differ from mortises in that they are typically cut perpendicular to a visible face and receive a key driven from outside, rather than receiving a tenon that is prepared on the end of an adjoining piece. Keyholes are also distinct from dovetail slots, which are angled differently and serve a different locking principle. In modern carpentry, keyholes are less common than they were in timber-frame building, as bolts, screws, and metal plates have become standard; however, they remain in use in heritage restoration, large-span timber structures, and where traditional joinery is specified.

The term keyhole reflects the shape and function: the cavity resembles a keyhole in a door lock, and the wedge action is analogous to how a key engages in a lock. A poorly cut keyhole, one that is too shallow or too wide, will not hold the key securely and the joint will fail under load. A keyhole cut at the wrong angle or with insufficient depth will cause the key to splinter or the timber to check around the opening.