The 1/3 rule and why it matters
Most woodworking references say the tenon should be 1/3 the thickness of the stock, no more. The reason is mechanical: a tenon that's 1/3 of the stock leaves 1/3 of solid wood on each side of the mortise, which is enough to resist the splitting force when the tenon is wedged or loaded in tension.
Going beyond 1/3 (say, 1/2) might look beefier, but it weakens the surrounding rail — the rail can split along the grain line at the mortise. Going below 1/4 produces a flimsy tenon that snaps under load.
Through, blind, haunched, wedged
- Through tenon: Visible on both faces. Strongest because you can wedge from both sides. Use for table aprons, chair stretchers, bed rails.
- Blind tenon: Hidden in the mortise. Weaker than through because you can't wedge from both sides, but invisible. Standard for face frames and cabinet doors.
- Haunched tenon: Has a small "haunch" step at the shoulder that fills the groove in a frame-and-panel construction. Prevents the rail from twisting. Standard for cabinet doors with floating panels.
- Wedged through tenon: A through tenon with a slot at the end that flares when a wedge is driven in. Locks the tenon in the mortise. The strongest variation, but visible.
Hollow vs square mortise
By machine:
- Squaring up with a router: Use a fence and straight bit. Faster, but you lose some tear-out control at the ends.
- Hollow mortise (Festool Domino or similar):strong> Floating tenon with a machine-cut mortise. Fast, accurate, repeatable. The floating tenon is essentially a loose tenon that glues into both pieces.
By hand:
- Chop and pare: Drill out the bulk with a mortising drill (or brace and bit), then square the walls with a chisel. Slower, but you control every cut.
Draw-boring for no-clamp joinery
Draw-boring is a centuries-old technique where you drill a pin hole through the tenon, assemble the joint, mark where the pin hits the mortise wall, re-drill the mortise 1/32" closer to the shoulder, then drive a hardwood pin through both. The pin pulls the tenon home as it goes in — the joint locks tight without clamps and can be assembled dry.
This calculator gives you the standard 1/32" offset. Adjust based on test fit: if the joint is loose, increase the offset; if it splits the mortise wall, decrease it.
How to cut a tenon by hand or machine
- Mark the shoulders on the end grain with a knife and square. Don't trust a pencil line.
- Set the tenon saw to the cheek cut first. Saw on the waste side of the line, leaving the line on the stock.
- Cut the cheeks: Saw down to the shoulder line on both sides. Use a bench hook for control.
- Cut the shoulders: Flip the workpiece, lay it flat, and saw across the shoulder line on the waste side.
- Trim with a shoulder plane for a perfect fit. Test-fit frequently.
Common questions
My tenon is twisted. What went wrong?
Either the shoulders aren't square to the cheeks, or the cheeks are different thicknesses. Lay the tenon flat on a known-straight surface and check for daylight at the shoulders.
Should I round over the tenon edges?
Lightly, yes. A 1/64" chamfer or roundover on the tenon's leading edges makes assembly much easier and helps the glue squeeze out evenly. Don't round over the shoulders — that's your reference surface.
What's the difference between a tenon and a floating tenon?
A traditional tenon is part of the workpiece (cut directly from the rail). A floating tenon is a separate piece of wood (often plywood or hardwood) glued into mortises in both workpieces. Floating tenons are faster with machines and allow angled or offset joints that would be hard to cut traditionally.