In this the edge of each board is cut into a series of alternate projections and indentations, known as the "pins" and "sockets," which fit one another and form the joint. In Fig. 490 the pins are formed upon A and the sockets on B.
The ends of the projections or dovetails show on each side of the angle formed by the boards when they are put together.
In some cases the spaces between the pins are only equal in size to the pins themselves, as shown in Fig. 490. This makes the strongest joint, but very frequently the pins are placed much farther apart.
The common dovetail is chiefly used for the angles of drawers and superior boxes, where they are generally not seen, but it is also occasionally adopted in building for securing the angles of skirtings, and for casings of a superior description.
Mitred, or Secret, Dovetails and Lap Dovetails are modifications of the above, used chiefly by cabinetmakers; they will be described in Part II.
Mortise and Tenon Joints, used for framing in joiners' work, resemble those in carpentry, but are much smaller, and require to be made with greater care and exactness, so that they may fit smoothly in all their parts.
The thickness of the tenon varies from 1/3 to 1/4 of that of the framing, care being taken to leave sufficient substance in the cheeks of the mortise. The width of the tenon should not be more than 5 times its thickness, or it will be liable to bend.
Haunching a tenon is the cutting away a part of it, so as to leave a piece (h, projecting to a distance of only 1/2 inch or 1 inch) between it and the outer edge of the rail on which it is formed. This haunch gives the tenon great lateral strength, and saves cutting so large a mortise hole. The haunch and the mortise to receive it may extend to the outer edge of the pieces framed together.
Fig. 491. Examples of haunching are shown in the rails of the doorr Figs. 513, 517.
Double Tenons are formed on very wide rails in framing. They prevent the rail from twisting, do not shrink so much as a single wide tenon, and do not require so large a mortise, which latter tends to weaken the framing in which it is formed. As the wood between the roots of the tenons shrinks more across the grain than the wood between the mortises does with the grain, the result often is to split the rail. The space between the tenons is haunched, as will be seen in Fig. 492 by examining the mortise.
An example of such haunching is also shown in the lock rail of the doors, Figs. 513, 517.
Occasionally two tenons side by side in the thickness of the framing are advisable, as, for example, in the lock rail of a thick door, to receive a mortise lock (see M, Fig. 513); but where a single tenon with cross tongues can be used, it is stronger and more easily fitted.
Stump Tenons or Stub Tenons are required if the frame be very thick as well as wide. They are tongues or projections left in the wood on each side of the tenon.
Slip feathers or cross tongues inserted in ploughed grooves are frequently used for the same purpose, as shown at x x in Fig. 492.
Housing consists in letting the whole end of one piece of timber for a short distance into another (see p. 70). The recess formed in one piece to receive the end of the other piece is called the housing, and one piece is said to be housed into the other. Fig. 493 shows the housings formed in the string board of a stair to receive the ends of the steps which are housed into it. (See Part II.)