Dovetails: Why They Point One Way, and What That Buys You

Before marking anything, decide which direction the joint will be pulled apart in service. The tails go on the piece that is being pulled, and they are angled so that pull drives them deeper into the pins. A drawer: tails on the sides, pins on the front, because the front is pulled. A carcase: tails on the piece that the load hangs from. A box with no obvious direction: either, and then it is chosen for appearance. Get it backwards and you have a joint that is beautiful, difficult, and no stronger than a rebate. It is the one decision in the whole joint that cannot be corrected afterwards.

The usual angles are about 1:8 for hardwood and 1:6 for softwood — roughly 7 and 9.5 degrees. They are not arbitrary and they are not decoration. A STEEPER angle (1:5 and beyond) gives a stronger mechanical lock but leaves the tips of the tails as very short, fragile, nearly-end-grain corners that break off during assembly. A SHALLOWER angle (1:10) gives robust tails and less lock, tending towards a finger joint, which has none at all. Softwood gets the steeper angle because its fibres are weaker and it needs more mechanical interlock; hardwood gets the shallower one because it can hold a fine tip and does not need as much. Which is the same trade as chisel bevel angles: the angle follows the material.

Cut the tails first, then stand them on the pin board and knife round them to mark the pins. Never measure both independently — the second set is transferred from the first, so any error in the first is copied rather than added to. That is the same principle as measuring everything from one face side: share a reference and errors stop compounding. A dovetail cut from two independent measurements is the cumulative-error rung producing gaps. Saw on the WASTE side of every knife line, and leave the line itself. The line is where the joint closes, and a saw kerf taken out of it is a gap that has to be filled.

Push the joint together by hand until it is about a third home, look at where it is binding, and pare THAT. Do not drive it fully home to see whether it fits: wood compresses, and a joint assembled dry is looser every time afterwards. Pare the pins rather than the tails when correcting. A pared pin is hidden inside the joint; a pared tail shows on the outside face. A dovetail is also almost pure long-grain glue surface on the tail cheeks, so a well-fitted one is both mechanically locked and properly glued. That is why it is worth the difficulty — it is the only common joint that solves the end-grain problem twice over.