Definition of "mortice"

One soon learned, however, that it was no easy thing to make wood do exactly as one required it. It looked quite easy in the masterly hands of the experienced pattern-maker, but the mysteries of dowelling and mortice and tenon were not uncovered in a week or two.

1944 November and December, A Former Pupil, “Some Memories of Crewe Works—II”, in Railway Magazine, page 341