The tenon was the tongue of wood that fit into the mortise. A rectangular peg carved from the end of one board that slid into a rectangular hole cut in another. The tenon and the mortise were the oldest joint in woodworking. Older than nails. Older than glue. The tenon was the board's handshake. Two pieces of wood agreeing to hold each other without help from anything but themselves.

The tenon was cut by hand. A backsaw and a chisel and a steady eye. The carpenter marked the line and cut to the line and the tenon was exactly the width of the mortise because exactly was the only option. A loose tenon was a wobbly chair. A tight tenon was a chair that lasted a hundred years. The difference between loose and tight was a thirty-second of an inch. The carpenter's reputation lived in that thirty-second.

The tenon did not need glue. A well-cut tenon fit so tight in the mortise that glue was insurance, not structure. The Shakers built chairs with dry tenons and the chairs are still in museums two hundred years later. No glue. No nails. No screws. Just wood holding wood because the carpenter understood that precision is its own adhesive. The machine-cut tenon needs glue because the machine cuts to a tolerance. The hand cuts to a fit.

The tenon swelled when it got wet. That was a feature. Furniture makers left the tenon slightly undersized and then exposed it to steam and the steam swelled the wood and the swollen tenon locked into the mortise so tight you could not pull it apart. The carpenter used the wood's nature against itself. The wood wanted to swell and the carpenter said then swell here. The screw does not swell. The screw does not want anything. The screw just holds.

They do not cut tenons anymore. The modern joint is a pocket screw driven by a jig at an angle through one board into another. The pocket screw takes thirty seconds and the tenon took thirty minutes and the pocket screw holds the same weight. But the pocket screw is hidden because the pocket screw is ugly. The tenon was visible because the tenon was beautiful. The carpenter who cut the tenon signed his work with the joint. The man with the pocket screw hides his work with a plug.

See also: Purlin, Joist