The one-third rule, and what it is actually protecting
Cut the tenon to a third of the rail thickness. On three quarter inch stock that is a quarter inch, on inch and an eighth stock it is three eighths. The rule is not about the tenon's own strength; it is about balance. Make the tenon thicker and the cheeks left on the rail get thin and the mortise walls in the leg get thinner still. Make it thinner and the tenon itself becomes the weak part. A third puts the failure point roughly where you want it and leaves enough wood on both sides of the joint to survive being cut.
In practice the rule bends to the tools. If your smallest mortise chisel is a quarter inch and the arithmetic says 0.267, you cut a quarter inch tenon, because a mortise that matches a chisel you own is worth more than a sixty-fourth of theoretical optimum. That is why this page offers to round to a size from a set rather than to an abstract fraction.
Length is limited by the leg, not the rail
A tenon can only be as long as the leg gives it. For a stopped mortise the convention is around two thirds of the leg thickness, which leaves wood behind the mortise and keeps the leg from becoming a shell. Going deeper buys a little glue area and costs structural wood in the part that has to resist splitting.
At a corner, where two rails meet in the same leg from perpendicular directions, the two mortises share the leg's interior and will collide if both tenons are long. The two conventional answers are to mitre the ends of the tenons so they meet at forty-five degrees inside the leg, or to make one shorter than the other. This page reports how much wood is left behind the mortise so you can see the collision coming rather than discovering it with a chisel.
Shoulders, haunches and what each one is for
The edge shoulders set the tenon in from the top and bottom of the rail. They give the rail a clean line where it meets the leg, they stop the mortise from having to run out at the leg's edge, and on a wide rail they let the mortise be shorter than the full rail width so the leg keeps more material. A quarter inch is a common figure on furniture-scale work.
A haunch is a short stub of tenon left at the top, usually where a groove for a panel runs along the inside edge of both the leg and the rail. The haunch fills that groove where it would otherwise show as a hole, and it resists the rail twisting. It is not doing much structurally, so a haunch taking more than half the tenon width is a sign the proportions have drifted.
Cutting the rail to length
The number people get wrong is the overall rail length. What you know is usually the finished clear space between the legs, the shoulder-to-shoulder dimension, because that is what the design calls for. The board you cut has to be that plus one tenon length at each end. Get this backwards and you build a frame that is short by the width of two tenons, which on a table apron is well over an inch.
Cut a test joint in scrap of the same thickness before committing. The fit you want on the cheeks is one that goes together with hand pressure and stays there; a tenon that needs a mallet will squeeze the glue out and can split the leg, and one that falls in has nothing for the glue to bridge. When the frame parts are sized, the cut list optimizer works out how they come off your boards and the board foot calculator prices the stock. If the frame holds a panel, the wood movement calculator tells you how much room that panel needs to move in its groove.
Questions people ask
Why is a tenon one third of the stock thickness?
Because it balances the three things that can fail. Thickening the tenon strengthens it but thins the cheeks on the rail and the walls of the mortise in the leg, and in practice the mortise walls are what give way first. Thinning the tenon strengthens the leg but makes the tenon itself the weak link. A third puts all three at roughly comparable strength and has the practical advantage of landing on common chisel and bit sizes for common stock. Treat it as the default and depart from it deliberately: a light frame can carry a thinner tenon, and a joint carrying real load may want more glue area from a wider tenon rather than a thicker one.
How deep should the mortise be?
A little deeper than the tenon is long, typically by a sixteenth. The extra is glue relief. If the mortise is exactly the tenon length, glue collects at the bottom, has nowhere to go, and either holds the joint open at the shoulders or hydraulically splits the leg when you clamp it. The relief also gives sawdust somewhere to sit. This is a small detail that causes a disproportionate number of joints that will not close, and the symptom is a shoulder gap that no amount of clamping pressure will shut.
What happens where two mortises meet inside a leg?
They intersect, and if you have not planned for it the second tenon will not go in. At a table corner, two aprons enter the same leg from perpendicular faces and their mortises share the interior of the leg. The two standard answers are to mitre the ends of both tenons at forty-five degrees so they meet neatly, or to make one tenon shorter than the other so it stops before the crossing. Either works. What does not work is cutting both to the full two thirds and hoping. This page shows how much wood remains behind the mortise so the collision is visible before you cut.
Should the tenon fit tight or loose?
It should go together with firm hand pressure and stay put on its own, and it should not need a mallet. A joint that has to be driven squeezes the glue out of the cheeks as it goes, which is exactly where the strength lives, and it can split the mortise walls. A joint that drops in has a gap that glue is poor at bridging, since most wood glues are weak in a gap. Cut a test joint in offcuts of the same thickness before committing to the real parts, and remember that the fit changes with moisture, so a joint cut in a damp week and assembled in a dry one will not feel the same.
Is a loose tenon joint as strong as a traditional one?
Broadly comparable when it is sized the same way, because the strength in either case comes from long grain glued to long grain across the cheeks, and a loose tenon has two glued mortises instead of one. The proportions on this page transfer directly: the same one-third thickness, the same total length split between the two mortises, the same edge shoulders. What changes is the failure mode if you get it wrong, since a loose tenon has two glue lines to go wrong instead of one, and the grain direction of the loose tenon stock matters in a way that an integral tenon never has to worry about.