Rule 6 of 31 · Chapter II — Measure and Mark
Measure twice, cut once
Why this rule exists
Wood is a one-way material. You can always take more off, but you can't put it back, and a board cut short is either firewood or a humbling lesson in patching. Measuring twice costs you ten seconds; a bad cut costs you a board, a trip to the lumberyard, and the momentum of your whole afternoon. The old saw is old because it's true. Slow down at the mark and you speed up the project.
In practice
Take your measurement, then take it again from the same reference before you commit the blade to the wood. Read the tape square-on, straight down, not at an angle, to avoid parallax error. Confirm which side of the line is waste before cutting anything. For any part you'll cut more than once at the same size, set a stop block instead of re-measuring, which is both faster and far more consistent than trusting the tape and your eyes every single time.
When it doesn't apply
The better move is often not to measure at all. When one part must match another, hold them together and mark directly, or use the mating piece as your gauge. Direct transfer beats a number every time because it can't be misread. Save the tape for when you truly need an absolute dimension.