What is a ‘mortice and tenon’ joint?

Below is a mortice and tenon joint, (before assembly) the most robust joining method in timber we have to date, its been tested for hundreds and hundreds of years across all continents and in every place developed by mankind ! Left picture (far left)is the rectangular cutout in the sash’s stile -the mortice, and tenon to the right,. The photo on the right is a finished sash, you can see the end grain of the tenon that's been cut off flush-this is a through tenon. Unused in today's modern joinery, as it allows exposure to water if the paint seal is compromised. This was standard manufacturing method 40+years ago … and stuck together with wedges and animal glue.