In a gained joint, as in a case of shelves, Fig. 266, No. [29], [p. 179], the gain in the uprights does not extend quite to the front of the shelves, and there is a corresponding slight shoulder at the front end of the shelf, so that if the shelf and support shrink unevenly, no gap will be apparent.
A drawing-board, Fig. 280, [p. 186], is so made that it can shrink or swell without losing its flatness. Shingles when properly laid, can shrink or swell without the roof leaking.
6. Where feasible, undercut joined surfaces so as to give clearance on the inside and insure a tight appearance. But glued surfaces should be made to meet flat.
Illustrations of this principle are as follows: The inner end of the socket in a dovetail joint, Fig. 267, No. [48], [p. 180], may be undercut slightly so as to insure the pin's falling close into place.
The shoulder of any tenon may be undercut so as to allow the edges of the tenoned piece to close up tight against the mortised piece.
In an end-lap halved joint, Fig. 265, No. [17], [p. 178], the edges should meet all around; if they are to be glued together, they should not be undercut or they will not glue well.
In matched flooring, the underside of the boards is slightly narrower than the upper side so that the joint may close on the upper side without fail, Fig. 301, [p. 201]. The ends of flooring boards are also slightly beveled so as to make a tight fit on the upper side.
7. Select the simplest form of joint and use the smallest number of abutments (bearing surfaces) possible, because the more complicated the joint or the greater the number of bearing surfaces, the less likelihood there is of a sound and inexpensive construction.
Illustrations of this principle are as follows: Usually a single mortise-and-tenon joint is better than a double one because of simplicity, strength and ease of making. Where much surface is required for gluing, a double one may be better.