The strongest, most traditional joint in furniture making. Sizing, layout, cutting, and gluing — start to finish.
The mortise and tenon is the workhorse joint of furniture making. A rectangular pin (the tenon) on the end of one piece fits into a matching hole (the mortise) in another. Done well, it is mechanically locked, glue-rich, and lasts centuries. Done poorly, it racks within a season.
PROPORTIONS
The classic rule of thumb is one-third stock thickness for the tenon. In 3/4" stock, that means a 1/4" tenon. This leaves equal cheeks on each side, which keeps the mortise walls strong and the tenon stiff. For 4/4 nominal hardwood (typically 13/16" actual), a 5/16" tenon is reasonable.
Tenon length should be at least two-thirds of the joined piece's width, capped at about 1-1/2" so you do not punch through the far side. For a chair stretcher into a leg, a 1" tenon is typical. For a table apron into a leg, 1-1/4" or stub it at 1" if the leg is narrow.
LAYING OUT
Mark the tenon shoulders all the way around the workpiece with a knife, not a pencil. The knife wall gives your shoulder plane a wall to register against and reads more accurately than a 0.5mm line. Mark the cheeks with a mortise gauge set off the chisel that will cut the mortise — that way the gauge and the tool agree.
CUTTING THE MORTISE FIRST
Always cut the mortise first, then fit the tenon to it. A mortise is harder to adjust than a tenon. With a mortise chisel, work from the middle out, drive the chisel straight down, and pry waste up between cuts. With a router, use a spiral upcut bit and a fence. With a hollow-chisel mortiser, take light bites and keep the chisel sharp.
CUTTING THE TENON
A band saw cuts tenons quickly: stand the workpiece on end with a fence set off the cheek line, then nip the shoulders flat on the table. A table saw with a tenoning jig works the same way. By hand, saw down the cheeks with a tenon saw, then saw the shoulders. Pare to the line with a wide chisel using the mortise gauge marks as a registration wall.
FITTING
A well-fit tenon slides home with hand pressure and a small mallet tap. If it falls in, it is loose — wedge it or move on. If you cannot start it by hand, pare cheeks (not shoulders) with a router plane or a sharp chisel held flat. Test the fit dry, then once more before glue-up.
GLUING
Spread glue inside the mortise and on the tenon cheeks. Hide glue, PVA, and polyurethane all work. Clamp across the joint to close the shoulders, not in line with the tenon — clamping along the tenon does nothing.
COMMON VARIATIONS
Through-tenons let you wedge from the back side. Haunched tenons add a small step to fill the groove that would otherwise show on a frame-and-panel door. Drawbored tenons use a slightly offset peg to pull shoulders tight without clamps — the traditional way to assemble a chair.