The “mortise” is the recess where the tenon goes. It can be cut by hand with a mallet and mortising chisel, or mechanically with a mortising machine and a hollow chisel bit. If you don’t have those ...

One common joinery method used in wood working is the mortise and tenon. A mortise is basically a hole in a piece of wood and the tenon is another piece of wood cut to tightly fit in that hole.