It is scarcely necessary in this work to say much on this part of joining timber or boards, it being analogous to that of morticing and tenonding. I shall therefore under this head, simply state, that when we wish to join two boards together, by means of a tongue and groove, the groove should never exceed one-third of the thickness, and often if the piece which is formed for the tongue is hard wood and not liable to split, one quarter of the thickness will be sufficient: or in the case of a panel let into a groove in the style, we must be often guided by the thickness of the panel itself, which should never be less than one-third the thickness of the style.

In making a groove across the grain for partitions, etc, it will be best in most cases to make the groove about a fifth or sixth of the substance of the stuff; but if the groove is formed into a dove-tail, one quarter the thickness will be best, and the dove-tail should be made a very small degree tapering, but not too much, and only so as to go almost home without requiring a blow from the hammer or mallet, to drive it into its place till very nearly so, as alt joints should only fit so tight, that before they are glued, they can be easily separated with a gentle blow. With regard also to a lap joint, or lapping two pieces together, supposing them of equal thickness, half the substance of each should be cut away; and if of unequal thickness, we should make the lap in the thinner piece, about two-thirds or three-quarters of its thickness, according to the substance of the thicker piece, thus endeavouring in this as in all other cases, to avoid weakening one piece more than another.