The Mortise and Tenon: What Each Part of It Is Actually For

Both tenon cheeks and both mortise walls are long grain. Glued together they make the strongest bond wood offers, and they are the reason the joint holds at all — the butt-joint rung is the argument for why. So the cheeks are where the fit matters. They want to be a firm sliding fit: close enough that the glue line is thin, loose enough that the joint can be pushed home by hand or with light mallet taps. Make the cheeks by sawing to a gauged line and paring, and check the fit dry before glue goes anywhere near it. A tenon that is too thick can be pared; one that is too thin needs a shim glued on, and that is a repair rather than a joint.

The shoulders are what the rail actually sits against, so they set the angle between the two pieces and they are what a racking force pushes on. A shoulder that is not square makes the frame out of square, and no amount of clamping fixes it. Both shoulders must be in the same plane and cut from the same gauge line. Cut one 0.5 mm deeper than the other and the rail twists — and it twists whether the cheeks fit or not. A visible gap at a shoulder is the joint's most obvious fault, which is why it is worth cutting the shoulder slightly proud and paring to the line with a chisel, rather than sawing straight to it and hoping.

Cut the tenon 2 to 3 mm shorter than the mortise is deep. That gap holds squeezed glue, lets the joint close fully on the shoulders, and costs nothing — the end grain at the bottom was contributing nothing anyway. A tenon that bottoms out is the classic bad joint: it holds the shoulders open by a hair, so there is a visible gap at the most visible place, and the cheeks may not even be seated. Proportions worth starting from: tenon thickness about ONE THIRD the rail thickness, so the mortise walls and the tenon are all about equally strong. Tenon width up to about five times its thickness — wider than that and it is usually split into two tenons with a haunch between, precisely because a wide tenon moves across its width and a long mortise weakens the stile.

Bore a hole through the mortise walls, assemble, and mark where the hole falls on the tenon. Then bore the tenon's hole about 1 mm CLOSER to the shoulder than the mark, and drive a tapered peg through. The peg is forced to bend as it passes, and pulls the tenon's shoulder hard against the stile — permanently, without clamps, and without waiting for glue. It is how timber frames and much pre-industrial furniture were assembled, and it works on green timber where glue does not. Offset too far and the peg snaps or the tenon splits; 1 mm is about right in hardwood, a little more in softwood. Make the pegs from riven, straight-grained stock — a sawn peg with grain running out will shear as it bends.