58.
It is common to loosen up the wood by first boring a series of adjoining holes whose centers follow the center-line of the mortise and whose diameter is slightly less than the width of the mortise.
Take care to bore perpendicularly to the surface, see Fig.

137, p.

86, and no deeper than necessary.

Dig out the portions of wood between the auger holes and chisel off thin slices, back to the gage-lines and to the knife-lines, taking care all the time to keep the sides of the mortise perpendicular to the face.

This may be tested by placing the chisel against the side of the mortise and standing alongside it a try-square with its head resting on the surface.
Finally test the tenon in the mortise noting carefully where it pinches, if anywhere, and trim carefully.