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. 199. 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.
In a dovetail dado, Fig. 266, No. 28, p. 179, it is usually sufficient to make the dovetail on one side only.
Many very elaborately spliced joints have been devised, which have no practical advantage over the simple ones, Fig. 264, Nos. 4-7, p. 177.
A butt joint, Fig. 264, No. 11, is stronger than a mitered joint, Fig. 268, No. 52, in a box, for the latter is almost sure to shrink apart. Where appearance is important, a ledge and miter joint has the advantage of both, Fig. 268, No. 58.
8. _Keep a due proportion of strength between the fastenings (joints) and the pieces fastened: i. e., the construction should neither be frail on the one hand, because the pieces of wood are weakened by too much cutting, nor clumsy, on the other hand, because then the fastenings would be inordinately strong. In other words, the different parts should be equally strong._
Illustrations of this principle are as follows: In a fished joint, Fig. 264, No. 2, the plate should be attached so as to reinforce the splice at the weakest point.
In a scarf joint, Fig. 264, Nos. 5 and 7, the angle should be oblique enough to give the greatest leverage.
In a tusk tenon, Fig. 267, No. 40, the tenon is made but one-sixth the thickness of the timber, whereas the tusk is made much larger.