Why should I mark with a knife instead of a pencil?

Line width, severed fibres and a physical wall for the tool — what a knife line does that a pencil line cannot, and when a pencil is still right.

Difficulty: beginner
Time: 12 min
Read: 3 min

Short answer

A pencil line is up to half a millimetre wide, so cutting to it is guesswork. A knife line has no width, it severs the surface fibres so cuts and chisels do not tear, and it gives the tool a physical groove to register in. Use a pencil for rough marking and a knife for anything that has to fit.

Joinery accuracy is mostly marking accuracy. A saw cut can only be as good as the line it follows, and the difference between a joint that closes and one that shows is often exactly the width of a pencil line.

What you'll need

marking knife or a sharp craft knife
try square or combination square
pencil for rough work

Cutting on the waste side of the line

Step by step

Use a pencil for layout and a knife for cutting lines.Rough dimensions, waste marks and notes belong in pencil. Any line a tool will follow should be knifed.
Understand what the knife does.It defines the position with no width, severs the top fibres so the saw or chisel cannot tear them, and cuts a groove the tool can drop into.
Hold the knife bevel towards the waste.The flat face runs against the square, so the line sits exactly at the square's edge and the bevel pushes the waste side away.
Score lightly first, then deepen.One firm pass often skates or follows the grain. Two or three progressively deeper passes give a clean, accurate line.
Mark the waste side clearly.A pencil cross or a squiggle on the waste. It is the simplest way to avoid cutting the wrong side of the line, which is the most expensive error in the workshop.
Cut on the waste side, leaving the line.The saw kerf takes 1 to 3mm. If the kerf straddles the line, the piece finishes short by half the blade width.
Transfer knife lines rather than remeasuring.Hold the actual mating part in place and knife directly from it. Every measurement you avoid is an error you avoid.
Drop the chisel or saw into the groove.For a chisel especially, the knife wall registers the blade and stops it drifting. That is why hinge recesses and joints are knifed rather than pencilled.
Darken a knife line if you cannot see it.Run a pencil along the groove and wipe the surface. The graphite stays in the cut and makes the line easy to see without adding width.

Tips

Pencil the waste side. It costs a second and prevents the most expensive mistake available in woodwork.
A pencil rubbed along a knife line makes it visible without widening it.
Knife lines only show on a finished surface if they are on the waste side or will be planed away — keep them where they will disappear.

Common mistakes

Cutting down the middle of the line — Half the kerf comes out of the piece you are keeping, so it finishes short. Over several parts the errors accumulate into a visible problem.
Using one heavy knife pass — The blade follows the grain rather than the square and the line ends up wandering. Two or three light passes track the square exactly.

If it doesn't work

Parts consistently a millimetre or two short

Cause: Cutting on the line rather than the waste side — Fix: Mark the waste, and leave the whole line visible on the finished piece.

Knife line wandered off the square

Cause: Too much pressure on the first pass, or the knife following the grain — Fix: Light first pass to establish the line, then deepen it.

Knife line visible on the finished surface

Cause: Line placed on the keeper rather than the waste — Fix: It can often be sanded or planed out, but plan the line position so it is removed by the cut.

Chisel still wandering despite a knife line

Cause: Line too shallow to register in — Fix: Deepen it, and pare a small chamfer on the waste side so the chisel can drop into the wall.

Questions people ask

What knife should I use?

A single-bevel marking knife is ideal because the flat face rides the square exactly. A sharp craft knife works well and costs little; a blunt one is worse than a pencil.

Does this apply to sheet material?

Yes, and especially to veneered and laminated boards, where the knife severs the surface film before the saw reaches it and prevents chipping.

When is a pencil better?

For rough cutting, for construction timber, for notes, and for any line you will need to see from a distance or rub out later.