20. All joiners' work that is not framed or "built up" should be so fixed that it is free to expand and contract. In a broad surface, such as a plain wainscot, this is accomplished by securing one edge only and permitting the other to rest in a groove, but in such a position that it may contract as the wood shrinks, or expand when affected by-dampness.

All wood is subject to a change in width across the grain, and when both edges are secured, shrinkage will cause the wood to split, and when dampness causes it to swell, it will either bulge in the middle or push the securing nails or screws out of place.

It may here be noted that a thorough knowledge of the properties of various woods is even more important in joiners' work than in carpentry. The slightest tendency of the material to warp, shrink, or swell must be fully compensated or prevented; for, however well seasoned the wood may be, it will always shrink somewhat across the grain when exposed to the warm, dry atmosphere of a dwelling house, and unless properly secured, thin panels will surely warp. For this reason, narrow boards are always to be preferred to wide ones, as the shrinking is distributed over a larger number of joints and is less perceptible; and, if the narrow strips are properly placed with respect to their original position in the tree, the tendency to warp may be largely compensated.