The Knife Line: A Mark You Can Put a Chisel Into

Use a knife for anything that will be cut to: shoulder lines, joint boundaries, the exact end of a piece. Use a pencil for everything else: which side is waste, part numbers, rough positions, notes to yourself. The reason for the split is that a knife line is permanent. It is cut into the wood and it can only be removed by planing the surface away, so putting one in the wrong place is a real problem — while a pencil line rubs off. So: measure and mark in pencil, check it, then commit with the knife. The two-step habit costs seconds and prevents the one mistake a knife makes worse.

A marking knife has a bevel on one side and is flat on the other. Hold the FLAT face tight against the square's blade, so the bevel pushes the waste side away and the cut sits exactly where the square is. Turn it round and the bevel wedges the knife away from the blade, so the line ends up a fraction into the good side. That fraction is the difference between a joint that closes and one with a visible gap. This is also why marking knives are made in handed pairs, or with a single bevel that you flip depending on which side of the line the waste is. Work out which side is waste before you cut — the sawing rung makes the same point about the kerf.

Cut the line in two or three light passes rather than one deep one. A heavy first pass follows the grain and wanders off the square; a light scoring pass creates the groove, and subsequent passes deepen it exactly where it is. Carry the line all the way round the workpiece, working from the face side and face edge each time. When you come back to where you started, the two ends must meet. If they do not, the stock is not square or the square is not — and you have found that out before cutting anything. That check is free and it is the reason marking out is done all the way round rather than just on the face you are about to saw.

For a shoulder that must be crisp, pare a shallow sloping trench on the WASTE side of the knife line with a chisel. That leaves a vertical wall exactly on the line for the chisel or saw to register against. Now the tool cannot wander into the good side: it is physically stopped. A chisel dropped into that groove finds the line by itself, which is what makes the difference between a shoulder that is nearly right and one that is exactly right. Keep the trench entirely in the waste. Its only purpose is to give the tool a reference, so it should never touch the finished side of the line — and it disappears entirely when the waste is removed.