What is an M & T door?

The mortise and tenon joint has been used for thousands of years by woodworkers around the world to join pieces of wood, mainly when the adjoining pieces connect at an angle of 90°. In its basic form it is both simple and strong. Although there are many joint variations, the basic mortise and tenon comprises two components: the mortise hole and the tenon. The tenon, formed on the end of a member generally referred to as a rail, is inserted into a square or rectangular hole cut into the corresponding member. The tenon is cut to fit the mortise hole exactly and usually has shoulders that seat when the joint fully enters the mortise hole. The joint may be glued, pinned, or wedged to lock it in place.
 

Types of mortise and tenon
A mortise is a cavity cut into a timber to receive a tenon. There are several kinds of mortise:
Open mortise – a mortise that has only three sides.
Stub mortise – a shallow mortise, in which depth depends on the size of the timber; also a mortise that does not go through the work piece (as opposed to a "through mortise").
Through mortise – a mortise that passes entirely through a piece.
Wedged half-dovetail – a mortise in which the back is wider, or taller, than the front, or opening. The space for the wedge initially allows room for the tenon to be inserted; the presence of the wedge, after the tenon has been engaged, prevents its withdrawal. It is sometimes called a "suicide" joint, since it is a "one-way trip".
Through wedged half-dovetail – a wedged half-dovetail mortise that passes entirely through the piece.
 
 A tenon is a projection on the end of a timber for insertion into a mortise. Usually the tenon is taller than it is wide.
There are several kinds of tenon:
Stub tenon - a short tenon; depth depends on the size of the timber; also a tenon that is shorter than the width of the mortised piece so the tenon does not show (as opposed to a "through tenon").
Through tenon - a tenon that passes entirely through the piece of wood it is inserted into, being clearly visible on the back side.
Loose tenon - a tenon that is a separate part of the joint, as opposed to a fixed tenon that is an integral part of one of the pieces to be joined.
Biscuit tenon - a thin oval shape piece of wood, looks like a biscuit
Tusk tenon - a kind of mortise and tenon joint that uses a wedge-shaped key to hold the joint together.
Teasel tenon (also spelled teazle)- a term used for the tenon on top of a jowled or gunstock post, which is typically received by the mortise in the underside of a tie beam. A common element of the English tying joint.
Top tenon - the tenon that occurs on top of a post.
Hammer-headed tenon - a method of forming a tenon joint when the shoulders cannot be tightened with a clamp.
Half shoulder tenon- An asymmetric tenon with a shoulder on one side only. A common use is in framed, ledged and braced doors.
Generally the size of the mortise and tenon is related to the thickness of the timbers. It is considered good practice to proportion the tenon as one third the thickness of the rail, or as close to this as is practical. The haunch, the cut-away part of a sash corner joint that prevents the tenon coming loose, is one third the length of the tenon and one sixth of the width of the tenon in its depth. The remaining two-thirds of the rail, the tenon shoulders, help to counteract lateral forces that might tweak the tenon from the mortise, contributing to its strength. These also serve to hide imperfections in the opening of the mortise.