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Woodworking Math

Bridle joint

A bridle joint is an open mortise and tenon: one piece has a slot cut into its end, and the other has a tenon, or at a corner a matching tongue, that drops into the slot. Because the slot is open at the end, the joint can be cut entirely with a saw. It is used for frame corners and to join a rail across a leg.

Proportions

A tenon is traditionally about one third the thickness of the stock being mortised.

The slot and the tongue are usually sized like a mortise and tenon, with the middle part about a third of the stock and the two cheeks of the slot taking the rest. That is the traditional tenon proportion, stated as such.

Traditional rule, stated by Popular Woodworking, Fine Woodworking (Taunton Press) and Woodcraft

How it’s cut

On the table saw a tenoning jig holds the piece on end to cut both the slot and the cheeks of the tongue; a dado stack can clear the slot in one pass. A band saw cuts the cheeks, and by hand a tenon saw and a coping saw or chisel do the whole joint. Cut the slot first, then fit the tongue to it.

Bridle or mortise and tenon

The bridle shows end grain on two faces of the corner and is open on one side, so it is usually pinned or relies on glue on its large cheeks. A mortise and tenon is closed on all four sides of the tenon. Both join end grain to edge grain with interlocking parts.

One source: USDA Forest Products Laboratory, the authority for wood adhesive joints it tested

Related joints

Sources

  1. Popular Woodworking, Christopher Schwarz, "Tenons Rule! So Here Are the Rules on Tenons" (2006), read 2026-11-07
  2. Fine Woodworking (Taunton Press), Gary Rogowski, "Tenoning Strategies", Fine Woodworking #142 (2000), read 2026-11-07
  3. Woodcraft, John English, "The Much-Heralded Mortise and Tenon", read 2026-11-07
  4. USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021), ch. 10 "Wood Adhesives", End-Grain and End-to-Edge-Grain Joints, read 2026-11-07

Proportions marked as a traditional rule are shop tradition, not measurements. This site gives no joint-strength numbers: the published tests each used a different rig. How we source these numbers.