Haunched Mortise and Tenon

A haunched mortise and tenon is a mortise-and-tenon joint with a short stub of tenon left on the outer edge, above the main tenon shoulder. The stub is the haunch. It sits in a matching mortise chopped into the stile, in line with the groove that takes the panel. The joint is used almost exclusively in frame-and-panel work: cabinet backs, doors, wainscot, and any frame where a rail meets a stile and a groove runs between them.

The problem the haunch solves is simple. The stiles and rails carry a groove along their inner edges for the panel, and on most frames the stile groove runs right out through the end of the stile. A tenon at the end of a stile has to be set in from the end, because a mortise cut to the full width of the rail would be open at the end of the stile and hold nothing. But a narrowed tenon leaves the end of the stile groove as an open slot, and leaves the outer part of the rail with nothing to stop it twisting. The haunch fills that slot. It closes the groove, stiffens the rail against twisting, and keeps the end of the stile intact.

Square haunch and sloping haunch

Two forms are common. The square haunch leaves the stub rectangular, with a square shoulder where it meets the main tenon. The recess for it is a short, shallow pocket in line with the panel groove; on many frames it is simply the groove itself, which already runs to the end of the stile. This is the default for painted work, for backs, and for any frame where the end of the stile will be covered or is not on show.

The sloping haunch, sometimes called a hidden haunch, tapers the stub from the shoulder down to nothing at the outer edge. The mortise is correspondingly shallow at the outer end. The advantage is that the end of the stile shows a single continuous surface with no visible stub line, which matters on an exposed frame: a door stile, a face frame, a screen. The disadvantage is that the sloping mortise is harder to chop cleanly and the fit is less forgiving. A sloping haunch also removes less material from the stile end, which helps on narrow stiles where a square haunch would leave too little wood around the mortise.

Choose square for structure and speed, sloping for appearance on exposed edges. Both do the same job; the sloping version is a refinement, not a stronger joint.

Why the stile is left long

When chopping the haunch mortise, the stile is left long by a generous margin, and the extra length, called a horn, is trimmed after glue-up. The reason is that the mortise for the haunch sits very close to the end of the stile. Chopping it with the stile already cut to final length invites the chisel to blow out the end grain, splitting away the corner and ruining the stile. The horn backs up the end grain and takes the breakout instead. Cut the stile long, mark the final shoulder line on the horn, chop and fit the joint, glue up, and cut the horn off flush once the glue has cured. The same approach applies to the rail: leave the rail slightly long if the shoulder line is close to the end.

Cutting by hand

Mark the mortise from the haunch stub on the rail, not from a separate measurement. Stand the rail on the stile in its final position, transfer the haunch width and depth with a marking knife, and carry the lines around the stile with a square. The haunch mortise is shallow, usually no deeper than the main mortise, and often less.

Chop the main mortise first, then the haunch. Use a mortise chisel matched to the mortise width, and work from the centre out, levering gently to clear waste. On the haunch, chop from the face side towards the back to avoid breakout on the show face, and stop short of the horn. Pare the ends clean with a firmer chisel. The haunch mortise should be a snug fit on the stub, not tight: a haunch that has to be driven in will split the stile along the grain between the haunch and the main mortise.

Cut the tenon and haunch on the rail with a tenon saw, then pare to the knife line. The shoulder of the haunch must be square to the shoulder of the main tenon, or the rail will not pull up tight. Check with a small square before assembly.

Cutting by machine

On a table saw, cut the main tenon cheeks first with the rail on edge against a tenoning jig or a tall fence. Then reset for the haunch: raise the blade to the haunch depth, set the fence to the haunch width, and cut the stub with the rail flat on the table. Clean up the haunch shoulder with a shoulder plane or a chisel so it sits square to the main shoulder.

A router with a straight bit and an edge guide will cut the haunch mortise, but the guide must be referenced off the face of the stile, not the edge, or the mortise will wander. A hollow-chisel mortiser handles the square haunch well; the sloping haunch is usually chopped by hand or routed with a jig, because the taper is awkward to set up on a mortiser.

Cutting the haunch removes a small offcut from the end of the rail. Make those cuts with the rail held in a tenoning jig or against a miter gauge, never freehand, and never let the offcut be trapped between the blade and the fence, where it can be thrown back. Do not climb-cut the haunch on a router table; the cutter will grab.

Judging the fit

A good haunched joint goes together with hand pressure and no mallet, pulls up tight at both shoulders, and shows no gap at the haunch line. The haunch should bear on its shoulder, not on its end grain. Test the fit dry, before glue, and look at the joint from the end of the stile: the haunch should sit flush with the surface or slightly below, never proud.

A bad fit shows in three ways. A gap at the haunch shoulder means the stub is too long or the mortise too shallow; pare the stub, not the mortise. A loose haunch that rocks means the mortise is too wide; glue a shim or recut the stub. A split at the end of the stile means the haunch was driven in too tight or the horn was cut off before glue-up. Any of these is a fit problem, not a strength problem, and the fix is to adjust the joint, not to add glue.

Where it fails

The common failure is a split stile, caused by driving a tight haunch into a mortise that is too small, or by chopping the haunch mortise after the stile has been cut to final length. The split runs along the grain from the haunch to the end of the stile and is usually invisible until the joint is under load. Leave the horn on, chop the mortise before trimming, and fit the haunch with hand pressure only.

The second failure is a twisted rail. This happens when the haunch is cut too narrow or left short, so it does not fill the groove. The rail then has nothing to resist twist and the frame racks. The fix is to cut the haunch to the full width of the groove, not to the width of the tenon.

The third is a visible gap at the haunch on an exposed frame, usually from a sloping haunch cut at the wrong angle. Cut the slope to match the mortise, not to a nominal angle, and pare the stub to fit.

Haunched tenon options compared

The mistake most often made

The mistake is chopping the haunch mortise after the stile has been cut to final length. It happens because the stile is already dimensioned and squared, and cutting it long again feels like wasted work. The result is that the chisel has nothing behind the end grain, and the mortise breaks out through the end of the stile. The joint may still go together, but the stile is weakened exactly where the haunch needs support. The fix is to plan the horn into the cut list: add a generous horn to both ends of every stile that carries a haunch, and trim after glue-up. It is a layout decision, not a cutting skill.

Related joints

For the parent joint, see mortise and tenon. For the frame that the haunch serves, see frame and panel construction.