This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
When the visible junction of 2 pieces is required to be as close as possible and no great strain has to be met at the joint, it is usual to slightly undercut the parts, and give clearance on the inside, as in Fig. 450, which shows an enlarged view of a tongued and rebated heading joint in flooring. In pattern-making, the fillets which are placed at the internal angle of 2 meeting surfaces are made obtuse angled on the back, in order that when bradded into place the sharp edges may lie close, as shown in Fig. 451. The prints used by pattern-makers for indicating the position of round-cored holes are also undercut by being turned slightly hollow on the bottom, as shown in Fig. 452. This principle is adopted in nearly all cases where a close joint is a desideratum. Clearance must also be left iu joints of framing when a settlement is likely to take place, in order that, after the settlement, the abutting surfaces may take a fair bearing to resist the strain.