Bridle joint
Also called: bridle, open mortise and tenon, corner bridle, t-bridle, mitred bridle
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A bridle joint is a mortise-and-tenon joint worked in reverse: instead of a tongue fitting into a slot cut in the other piece, one piece is cut away in the middle so it straddles a matching tongue left standing on the other. That projecting tongue is called the bridle; the two pieces left standing on either side of the slot, which flank it, are its cheeks.
How it's cut and why it holds
To make the joint, a mortise, cut into the receiving post, is shaped to fit the bridle, the projection left standing on the mating beam. A related cut, the slot mortise, is essentially a deep dado, a rectangular channel worked across the grain, cut into the end of a part; it is used to form both slip joints, a related joint family cut the same way, and bridle joints, both relatives of the ordinary mortise and tenon. As a rule, the bridle's width should be kept well below the width of the beam it is cut into, or the cheeks may weaken and be wrenched off by lateral pressure. Against a full mortise and tenon, a bridle joint takes less work to cut, since it is a simplified version of one, and it generally fits more accurately and weakens the wood less than the alternatives, giving a stronger joint overall.
Where it's used, in framing and furniture
The bridle joint is used mainly in carpentry, structural framing work, as distinct from finer joinery, and it is favoured in heavy carpentry framing where timbers meet at a sharp, acute angle. Alongside dovetail, halving and housing joints, it is one recognised category of framing joint. Its simplest form is called a tongue-and-fork joint, also known as an open mortise and tenon; timber framers use it, among other places, to join sills at their corners and to close rafters at the apex, the peak where two roof timbers meet, since it is simple to cut and assemble there. In one style of roof framing, an oblique bridle joint can be used together with a pole-plate, a horizontal timber that carries the feet of the rafters. In furniture, where an angled chair seat rail meets a leg, the tenon is angled if that keeps the grain running continuously along its length, otherwise the mortise is angled instead. Where a leg is set in from the end of a piece, a form of bridle joint can join it; cut at the top of the leg this way, the joint lets the leg's grain run continuously through to the top, which Fairham (1920) said gives the finished piece a more workmanlike appearance. Where the middle stile of a door or frame, its vertical centre member, runs as one unbroken piece, a bridle joint lets the top and bottom rails wrap over it as continuous members too; a shaped bottom rail in one such frame was joined the same way.
Strengthening and cutting it today
Members can be joined with bridle joints reinforced by draw pins, pins driven through the joint to draw it tight, or, in some cases, with handrail bolts and dowels instead; adding a single pin to a bridle joint gives it good strength on its own. This doubled gluing surface and improved resistance to racking, the twisting force that skews a frame out of square, shows up in T-orientation laps, where one member meets another in a T shape, in bridle joints cut with slot mortises, and in ordinary mortise-and-tenon joints alike. Because the joint brings long grain, running along the fibres, up against end grain, the surface left where the fibres are cut across, its two faces contrast visually once assembled. Rivington (1904) recorded that Tredgold recommended the bridle joint, in all its forms, for practical use. A single table-saw accessory can today cut spline slots, bridle joints and tenons all with the one setup.