Fig. 589.
Fig. 590.
The length to which a mortise can safely be cut is also a matter of judgment according to circumstances. If the tenon is thin, the mortise can be longer than if the tenon is thick, as the cheeks will be thicker and stronger, but, as a rule, avoid trying to make very long mortises, unless the tenon is very thin and the wood very strong, as there will not be strength enough left in the cheeks of the mortise (Fig. 590). Six times as long as it is wide is about as long as it is well to make a mortise under ordinary circumstances, though, as just said, it all depends on the conditions of the particular piece of work.
Fig. 591.
Fig. 592.
When a wide piece is to be mortised into another piece, two or more tenons are sometimes cut, thus avoiding too long a mortise, but this will not do for very wide pieces, unless some of the tenons are fitted loosely, for the expansion and contraction of the wide piece may cause it to buckle or split if all the mortises fit snugly (Fig. 591).
In such cases as a door-frame or when the end of a board is to be fitted into the side of a post, a tongue and groove is often used in addition to the tenon, and this (known as "relishing") is a good way to do (Fig. 592).