Boards are sometimes kept tight together at the ends by a "clamp" (C C, Fig. 157) running across them, grooved on the edge to receive a tongue left on the boards, which are thus free to shrink in width. In the best work tenons are also formed on the ends of the boards, which fit into mortises formed in the clamp.
In some cases the boards are cross-grooved, and the clamp tongued.
When it is advisable, for the sake of appearance, to conceal the ends of the clamp, they are mitred back, as shown at M M, Fig. .157.