48, p.

180, may be undercut slightly so as to insure the pin's falling close into place.
The shoulder of any tenon may be undercut so as to allow the edges of the tenoned piece to close up tight against the mortised piece.
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.