What you buy is not what you cut.

The rule, as it is used in the yard

The length allowance is millimetres and not a percentage, and that was learnt off a real cutting list. A percentage grows with the part: a 1415 mm foot came out as a 1600 mm blank nobody saws, when the sane answer is 1500 — two out of a 3000 mm board with nothing left over. A 2900 mm part asked for 3190 mm, which is not a length anybody sells.

• Length +50 mm, then rounded up to a full 50. 630 becomes 700. 1415 becomes 1500. 2900 becomes 2950.

• Width +10 mm. Always.

• Thickness +4 mm if the faces will be dressed, +1 mm if the part is turned — a turned part is cut round anyway. Then up to 26, 32, 40 or 50 mm. Nothing above that is invented.

The 23 mm trap

Rounded straight up, 23 mm reads as a 26 mm board. Add the 4 mm a planer takes and it needs 27, so the board is 32. On a chair that step is not a rounding error. It is the timber.

Sheets, glue, defects

A sheet of 1200 × 400 is bought as 1210 × 410 and keeps its own 18 mm. A chair seat of 480 × 440 is bought as 490 × 450. Defects are a percentage on the material, applied to the cubature at the end, never to the dimensions: a knot does not make the blank longer. A sheet has no knots, so the percentage is not offered.

When no sold board reaches the thickness, Estifex stops. Glue as a block (stacked in the thickness) or as a panel (across the width) is a decision the workshop makes, because gluing 3 × 180 instead of 4 × 110 costs more timber and that is their money.