This section is from the book "A Treatise On Architecture And Building Construction Vol2: Masonry. Carpentry. Joinery", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
Keys And Keying. When broad, plain surfaces, such as dados, window backs, table tops, etc., are required, the boards forming them are generally tongued-and-grooved together, or they are doweled and united with glue; but, to secure the surface against warping or twisting, pieces of wood called keys are let into a dovetailed groove in the back, as shown at a, Fig. 12. These keys are either planed off flush with the surface into which they are sunk, or, when extra strength is required, they may stand up from the back, as shown at b. They should not be glued in position, but are sometimes secured with a single screw or nail at one end, as shown at c, so that when the material shrinks it will slide upon the key, and the joint along the keyway or groove will still remain tight.
Clamping. Boards are sometimes maintained against warping by means of clamping, as shown in Fig. 13. The boards a and b are doweled or tongued-and-grooved together on their interior edges to form a close joint, as at c d, and the clamps h,j are glued over tongues worked on the ends of a and b, as shown at e.
In first-class work, tenons f are also worked on a and b, and by insertion in the mortises in the clamp h strengthen the joint materially. When it is undesirable to show the end grain of the clamp, as at e, or of the tenons, as at f, the ends of the clamps are mitered off as shown at g, and the tenons are not permitted to extend entirely through the clamp.
Tenons and mitered clamps can only be satisfactorily used when the material is thoroughly seasoned, and where the finished product will not be subjected to changes in temperature, as under these connections no provision is made for expansion and contraction.