If a piece of timber has to be framed in between two beams already fixed, it is evident that the tenons could not be got into ordinary mortise holes.

To enable the cross-piece to be fixed a chase is cut, as shown in Fig. 186, leading to the mortise, m, and the cross-piece is first held obliquely until the tenon enters the end of the chase at a, whence it is slid along into its place at m.

It may sometimes be necessary to make a vertical chase-mortise in a horizontal beam. This should, however, be avoided if possible, as it cuts through so many fibres. The mortise should be parallel to the grain of the timber.