This section is from the book "A Practical Treatise On The Joints Made And Used By Builders", by Wyvill J. Christy. Also available from Amazon: Practical treatise on the joints made and used by builders.
Mortise and Tenon Joint is formed by inserting a tenon into its corresponding mortise, so that the shoulders of the tenon bear upon the abutting cheeks of the mortise in vertical framing, whilst the side or edge of the tenon bears upon the cheeks or ends in horizontal framing. In either case the tenon should fill the mortise as completely as workmanship will permit; but when inserted horizontally into a level timber/ it is strengthened, that is to say, relieved of much stress, by a shoulder or tusk, Fig. 47, the mortise being cut in the neutral part or the middle of the depth of the beam. In almost every instance the tenon is secured by a pin, key, or treenail (excepting when straps or bolts are required), whose diameter equals about one-fourth the thickness of the tenon, and whose distance from the shoulders is about one-third the tenon's length. A tenon in its simplest form is always a parallelopipedon in shape, and is usually made by sawing out an equal-sized notch on opposite sides of the end of a timber in the direction of its grain, as in Fig. 48. The plane of junction of the tenon with the rest of the piece to which it belongs is called its root, and the surfaces on each side of the root its shoulders. The mortise, which is accurately gauged on another timber to the exact size of the tenon it is intended to hold, is hollowed out with a chisel with its sides or cheeks parallel to the grain, Fig. 49, and perpendicular to the plane of the mouth of the mortise. In carpentry, in vertical framing, the tenon and mortise are each one-third the thickness of the timbers joined, these being in this respect of equal dimensions; and the above description answers for the common case of joining at right angles, but sometimes the tenon and mortise have each one side notched or dovetailed, the remainder of the mortise not occupied by the tenon being filled up by a wedge. When the timbers are inclined to one another, the tenon is truncated, its shoulders bevelled, and the abutting cheeks of the mortise joggled as in Fig. 26, which shows the joint between a brace and post in a partition, or as in Fig. 44, which illustrates one of the numerous modes of uniting a principal rafter to a tie-beam. Double or parallel tenons are not much used in carpentry, the charge against them being that shrinkage and the imperfection of workmanship render it next to impossible to fit them so as to bear equally, whereby they not only thus weaken the framing, but produce an additional weakness by requiring two mortises side by side. Occasionally tenons are cut to fit mortises having a cross, L-shaped, or other form. The mortise and tenon is the fundamental joint of framing, though a frame or framing in carpentry is merely a collection of timbers united together by spikes, pins, or any means of connection whatever, and therefore not necessarily framed in the usually accepted meaning of the term, notwithstanding that it generally is. A truss is a frame having its parts so joined together by means of ties and struts that theoretically it cannot rack or change its form without rupture. When it consists wholly of timber its joints are almost always compound ones, consisting of 1 combination of the mortise and tenon with some sort o fastening, but when timber is in position moisture chiefly evaporates from the joints where the grain is cut across, so that a properly seasoned timber truss is far less likely to rack and fail than an unseasoned one.