Cut a Mortise and Tenon That Fits by Sizing the Tenon to the Stock, Then the Joint to the Mortise

The Problem

A mortise and tenon fails in two ways, and both are decided before any wood is cut.

Size it by eye and the tenon is either too thin to be strong or so thick the mortise walls are down to a shaving. Cut the tenon first and then try to chop a mortise to match it, and you spend the fitting stage removing material from the hard part — a mortise is an enclosed cavity, so paring its walls to chase a tenon that's already made means working blind against surfaces you can't see or reference.

The Technique

Sizing. Two proportions carry most of the work:

• The tenon is about one third the thickness of the workpiece. That leaves enough meat either side for the mortise walls to survive.
• The mortise is about two thirds of that same width, deep.

Order. Cut the mortise first, then fit the tenon to it. The tenon is the accessible part — four faces you can see, reach and shave a hair off. Fitting the easy part to the hard part is what makes this straightforward.

1. Lay out the tenon with a marking gauge, then lay out the mortise from it.
2. Do the whole layout in pencil first and check that everything reads right, front to back and edge to edge. Mistakes at this stage are free.
3. Once it's right, go back over the lines that will actually be cut with a marking knife. The knife line does two jobs: it severs the surface fibres so the edge stays crisp, and it gives you a physical wall to see and to register a chisel against.
4. Hog out the mortise waste — drill press or router — getting as close to the line as you can without crossing it. This is where the knife lines earn their place, because you can see exactly where the edge is instead of guessing at a pencil smear.
5. Pare to the knife lines to finish the walls.
6. Cut the tenon slightly fat and fit it to the mortise, taking material off in small passes until it slides home.