Mortise and Tenon Joints: Complete Guide

The mortise-and-tenon is the workhorse of furniture joinery. Properly cut and assembled, it creates joints strong enough to last generations.

Anatomy of the Joint

The mortise is a rectangular hole, typically cut into a thicker piece—a table leg or door stile. The tenon is a shaped projection on the mating piece that fits precisely into the mortise.

The tenon has four elements: two cheeks (the wide faces), two shoulders (where it meets the base piece), and sometimes one or two edges. The shoulders bear against the mortised piece, creating the visible joint line.

Proportions

Traditional rules of thumb: tenon thickness equals one-third the stock thickness. Tenon length equals five times its thickness, when possible. These ratios produce joints with adequate glue surface and mechanical strength.

Real-world conditions require adjustment. Thin rails might need thinner tenons. Short stiles limit tenon length. The formulas provide starting points, not rigid requirements.

Variations

Through Tenon

The tenon passes entirely through the mortised piece and shows on the opposite face. Often wedged for additional strength. Common in Arts and Crafts furniture.

Haunched Tenon

A notch at the tenon’s end fills a groove in the mortised piece, often where a panel groove runs through. Used extensively in frame-and-panel construction.

Tusk Tenon

A through tenon with a tapered hole that accepts a wedge. The wedge can be removed for disassembly. Traditional for knockdown furniture and workbenches.

Loose Tenon

Both pieces receive mortises. A separate tenon joins them. This simplifies cutting—all mortises can be machined identically—and works well with routed or biscuit-joined mortises.

Making Strong Joints

Fit matters most. A joint that slides together too easily lacks holding power. A joint that requires hammering risks splitting. The tenon should slide in with firm hand pressure.

Grain orientation affects strength. Tenon grain should run lengthwise, parallel to the tenon’s long dimension. Cross-grain tenons break easily.

Glue only the cheeks, not the end grain. End grain absorbs glue poorly and doesn’t contribute much to bond strength. The cheek-to-mortise-wall bond carries the load.