Tenon comes from the same root word as "tendon," "tenant," and "tenacious," all of which convey the sense of "holding." A tenon is any extension that fits into a socket or mortice in another piece to help hold the two pieces together. A tenon is usually a continuous part of one of the two pieces, but it may also be an independent part, or a "free" tenon. In this last sense, a dowel joining two boards is a free tenon. As you face forward, your cheeks are on either side of your face. It's the same with a tenon, a dovetail, or a mortice: the cheeks are the longgrain surfaces on the sides of the joint. If your head was a tenon, then your shoulders would be the shoulders of the tenon. Because a tenon is almost always cut at the end of a piece of wood, the shoulders of a tenon are almost always cut across the grain, exposing a face of end grain.