Cumulative Error: Why You Measure From One End, Not Piece to Piece

CHAIN measuring is marking each position from the previous mark. It feels natural and it is what a tape measure invites, because you keep moving the hook. ABSOLUTE measuring is leaving the tape hooked at one end and reading every position from there. Each mark has one measurement's worth of error and no more. The difference is invisible for two or three marks and serious for ten. It is why a shelf unit made by measuring each shelf from the last has a visibly uneven top shelf, and why a cutting list should give cumulative positions rather than individual lengths.

Clamp all the identical parts side by side with their ends flush and their face sides all the same way up, then strike every line across the whole bundle at once. Now the parts cannot disagree, because there is only one line. Any error in its position is shared, and a shared error usually does not matter — a shelf unit that is uniformly 1 mm shorter than the drawing is fine, while one with shelves at slightly different heights is not. The same reasoning gives the stop block. Set it once and cut ten pieces against it, and all ten are identical; measure ten pieces individually and you have ten slightly different lengths, however carefully you measured.

Decide which of the two you actually need before laying anything out, because the right method differs. If the OVERALL size must be exact — a piece fitting a known opening — measure absolutely from one end and accept whatever the divisions come to. If the DIVISIONS must be even and the total can be whatever it is, step them off with dividers and never measure at all. Most work is the second kind and gets treated as the first, which is why so much of it has a visibly narrow space at one end. Deciding this question explicitly is the whole of good layout, and it takes ten seconds at the start of a job.