The mortise and tenon has survived five thousand years because its proportions are self-balancing. Make the tenon too thin and it shears; too thick and the mortise walls that hold it become too thin instead. One third of the stock thickness is where those two failures balance.
The one-third rule and why it exists
Divide the rail thickness by three. That is the tenon, and it leaves a third of the thickness as a cheek on each side of the mortise.
The tenon has to resist being sheared off along the grain. The mortise walls have to resist being split outward. Those two strengths move in opposite directions as you change the tenon thickness, and a third is roughly where the total is strongest.
For three-quarter inch stock that gives a quarter inch tenon, which is why quarter inch is the most common mortise size in furniture. It is not a coincidence — it is the arithmetic landing on a convenient number.
Round to the tool you own
Inch and a quarter stock divided by three is 0.417 inches, and nobody makes a chisel that size. Round to three eighths or seven sixteenths, whichever you have.
The shoulders absorb the difference, and they do not care about being uneven by a thirty-second. What matters is that both shoulders are the same, which comes from setting a fence once rather than measuring twice.
A hollow chisel mortiser, a router bit, or a bench chisel all come in fixed sizes. Design the joint around the sizes in your shop and the work gets much easier.
Depth, and stopping short
Two thirds into the leg is the usual target. Deeper gets you little extra strength and risks breaking through the far face, especially on a leg that also takes a mortise on the adjacent side.
Where two mortises meet inside a leg — which happens at every table corner — the tenons have to be mitred or stepped so they do not fight for the same space. Work this out on paper before cutting, because discovering it at glue-up is too late.
The tenon itself should stop about a sixteenth shy of the mortise bottom. That void gives excess glue somewhere to go. A tenon bottomed out in its mortise acts as a hydraulic ram and will not seat.
Haunches and the width of the tenon
The tenon is normally set in from the top and bottom edges of the rail, leaving a small shoulder at each end. Without it, the mortise runs out at the top of the leg and becomes an open slot with no strength across it.
On a rail that also takes a groove for a panel, the top of the tenon gets a haunch — a short stub of full width that fills the groove where it passes the leg.
Width is what gives the joint its resistance to racking, so a tenon should be as wide as the rail allows. A long narrow mortise is stronger than a short wide one for the same area.
Glue on the cheeks, not the shoulders
The strength is in the cheeks, which are long grain to long grain. The shoulders are end grain against long grain and contribute almost nothing structurally, though they do close the joint visually.
So the glue goes on the cheeks and the mortise walls. Slathering the shoulders just produces squeeze-out to clean off an inside corner, which is the hardest place to clean.
A well-fitted joint should be able to be pushed together by hand and hold by friction before any glue. If it needs a mallet dry, it will split the leg wet — glue swells the wood.