A chair that wobbles is not a mystery. It's a shoulder that stopped bearing.
The mortise and tenon has been in furniture for several thousand years — Japanese tansu, Windsor chairs, Shaker case pieces, Danish Modern frames. That's not tradition. That's evidence. The joint keeps getting used because it answers the right questions about load. When you understand those questions, the proportions and the cutting sequence stop being rules to memorize and start being things that make sense.
Why it works
The problem the joint is solving is racking. In a frame — chair leg to rail, table apron to leg, door stile to rail — the load is sideways. The joint is not being pulled apart. It's being pushed sideways, repeatedly, for years.
A butt joint here relies on glue at end grain. PVA at end grain gets you 30–50% of full adhesion on a good day. In a chair, that fails fast. You've probably sat in one. You know what it sounds like.
The M&T does three things the butt joint can't. It creates long-grain cheek surfaces — the tenon cheeks and the mortise walls are both long grain, and PVA there achieves full strength. The shoulders bear racking load mechanically, wood on wood, independent of the glue. And the tenon depth seats the joint inside the adjoining member instead of hanging off the surface.
This is the argument. Not tradition. Physics.
Proportions
Tenon thickness: 1/3 of the stock at the tenon. A 3/4" rail gets a 1/4" tenon. A 1-1/2" rail gets roughly a 1/2" tenon. The logic: 1/3 remaining on each side of the mortise. Thin enough to cut cleanly, thick enough that the walls don't blow out. Err thicker in soft species — alder, soft maple — where the walls need mass. In dense, short-grain stock like quartersawn white oak or hard maple, you have more room.
Tenon length: 2/3 of the depth of the adjoining piece. A 1-3/4" square chair leg gets a tenon about 1-1/4" deep. Longer resists more racking; returns diminish well before you hit the opposite face, so don't go threading through if you don't have to.
Shoulders: the shoulder-to-shoulder dimension controls the joint's fit quality more than the mortise does. An open shoulder is not an aesthetic problem. It is a structural problem. The shoulder is load-bearing — it's what resists racking — and if it doesn't seat fully, the joint is relying on glue to do mechanical work. That never ends well.
One more: haunch tenons. If the rail enters a grooved stile — in a door, in a frame-and-panel — you need a haunch to fill the groove at the corner. Skip it and the stile will rotate out of the plane of the frame under load. It will do this slowly, then all at once.
Layout: mortise first
Always mortise first. The mortise is the fixed constraint; the tenon gets fitted to it.
Set your mortise gauge to your chisel — the actual chisel you plan to use, not a nominal measurement. Chisels vary from stated width. A mortise fitted to the chisel is straightforward to clean up. A mortise fitted to a measurement using the wrong chisel is a problem you create for yourself.
Mark tenon shoulder lines by knifewall directly from the mortised piece, with the parts clamped together. Don't transfer a measurement. Transferred measurements accumulate error. The knife line from the actual part does not.
Cutting
Two reliable methods for the mortise: chop with a mortise chisel or rout with a plunge router and straight bit.
Chopping: work center-outward, in passes. Don't lever against the end walls early — those walls are where the tenon bears, and you want them clean. Take thin passes. Pare the walls square on the final pass.
Routing: faster for multiples. Upcut spiral bit. Fence or edge guide for position, stop blocks for length. No more than 3/8" per pass in hardwood. You'll get radiused ends; square them with a chisel or round the tenon ends to match. Rounding the tenon ends is faster and structurally identical.
Neither method is inferior. Use whichever one you'll execute consistently.
For the tenon: cut shoulders first, to your knife lines. The shoulders determine what the joint looks like from outside. Then cut the cheeks to produce the tenon thickness. A tenoning jig at the table saw keeps multiples consistent. By hand: ripsaw or tenon saw down the cheek, just outside the line, then pare to fit.
Fitting: the tenon should seat with moderate hand pressure. Not a drive fit — that can split mortise walls when the glue swells things on final assembly. Not a sloppy fit — that's a glue joint, not a fitted joint. To find high spots, rub pencil on the tenon cheeks, insert partially, and read the shiny contact marks. Pare those. A shoulder plane on the cheeks. Creep up on it. You cannot put material back.
Glue-up
Glue goes on the mortise walls and the tenon cheeks. Not on the shoulders — they need to bear clean. Not on the tenon end grain — it absorbs glue and contributes nothing.
Assemble in stages if you have a complex frame. Two long rails to two legs first — clamp, check square, let it set. Then add the short rails. Easier to square in stages than all at once.
Clamp across the mortised member, pulling the shoulder into the mortise face. One clamp per joint if the fit is good. Two for wider rails.
Standard PVA gives you 5–10 minutes of open time. If you have a complicated multi-stage glue-up, use liquid hide glue — 15–20 minutes. Don't find out your glue has kicked halfway through assembly.
Drawboring is worth knowing: drill the mortise walls before assembly, drill the tenon slightly offset toward the shoulder, drive a tapered wood pin through the assembly. The offset creates tension across the joint without clamps. For joints you can't easily clamp — the underside of a table stretcher, the rear leg of a chair — this is the method.
What failure looks like
A racked joint: the shoulders have stopped contacting on one side. You'll see wobble and a visible gap. The cheek glue is often still intact — the joint didn't come apart, it just stopped bearing. Repair: clean the joint, re-glue the cheeks, reset the shoulders under clamp pressure.
A split mortise wall: the tenon was over-tight, or the walls were too thin to begin with. Below 1/4" of remaining wall, you're designing a failure. There's no structural shortcut on this one — you're patching or replacing the mortised member.
A shoulder gap that was never closed: a shoulder cut that wasn't square, or clamping pressure in the wrong direction during glue-up. The joint set with the gap in place. Glue didn't close it because glue doesn't close gaps. This is a cutting problem. In a finished piece it's sometimes acceptable aesthetically. Structurally it means the joint is relying on glue where it should be relying on bearing surface.
Know what the joint is solving. The proportions follow from the load. The sequence follows from the proportions. The fit follows from the sequence. All of it is downstream of one question: what is this joint actually resisting?
Racking. The shoulders bear it. The cheeks hold it. Cut both right.