The haunched mortise and tenon joint is a variant that adds extra strength against twisting forces. The extra haunch on the tenon give the rail piece a wider contact surface with the joint, without weakening the top of the stile piece or leg piece by opening up the end the way an open mortise and tenon or a bridle joint would.

This plan has a 3/4″ stock rail piece going into a 1″ (4/4) leg or stile piece.