mortise

A mortise is a rectangular hole cut into one piece of wood to receive a projecting tenon from another piece, creating a strong interlocking joint. The mortise is the female element; the tenon is the male element that slides into it. This joint, called a mortise and tenon, has been the backbone of furniture and timber framing for centuries because it distributes load across a large glued surface and can be wedged or pegged for permanent assembly.

The cavity itself is typically cut with a mortise chisel, a thick flat tool struck with a mallet, or increasingly with a hollow-chisel mortising attachment on a drill press. The walls must be reasonably square and parallel for the tenon to fit snugly. Depth varies with the application: a shallow mortise in a chair rail might be 1.5 inches deep, while a through mortise in a door frame (one that passes completely through the thickness) requires careful layout to avoid tearout on the exit side. Width and length are dictated by the tenon dimensions, usually leaving 1/8 to 3/16 inch of clearance to allow for finish and glue.

Common problems and variants

Mortises cut too deep or with walls that are not parallel reduce glue contact area and weaken the joint. A stopped mortise (one that does not pass through) is more common in case furniture and requires careful chisel work at the bottom to avoid splitting the surrounding wood. A through mortise, where the tenon protrudes through the far side, is typical in structural timber work and allows for visible wedging or pegging. Slip mortises, cut partway across a board without penetrating completely, are used when a visible hole through the face is unacceptable.

The name derives from Old French mortaise, possibly connected to the verb meaning 'to bite' or 'to gnaw', a fitting description of how a chisel tears into wood. In timber framing, mortises are often cut before tenons because the cavity can be laid out more accurately on the wider surface of the receiving member. In furniture production, the order depends on the design; some makers prefer to tenon first and fit mortises to the finished tenons rather than the reverse.

Mortise and tenon joints appear in chair seats, table aprons, door frames, and structural timber assemblies. They are stronger than dowel joints under shear stress and more reliable than pocket-hole joinery in hardwoods, though they demand more skill and time to execute. Modern production sometimes substitutes mortises with biscuits or dominos for speed, but the mortise and tenon remains the reference standard for strength and longevity in fine joinery.