Lock-Miter Joint

A lock-miter joint cuts a matched, interlocking S-shaped profile into the mating edges of two boards, using a single specialty router bit run on both pieces (one flat on the table, one on end against a tall fence). Assembled, the joint shows a clean 45-degree miter from the outside -- no visible end grain, the way a plain miter joint looks -- while the interlocking profile underneath adds far more glue surface and mechanical strength than a bare miter alone provides.

The catch is setup precision: the bit height and fence position both have to be dialed in almost exactly right, or the two profiles won't mesh and the outside faces won't sit flush. Most lock-miter setups take a few test cuts in scrap of the actual project thickness to get right, which is the main reason the joint has a reputation for being finicky despite being straightforward once dialed in.