Rule 10 of 31 · Chapter II — Measure and Mark
Square your reference before layout
Why this rule exists
All your careful measuring means nothing if the edge you're measuring from isn't straight and square. Layout radiates from your reference face and edge, and if those are off, every mark inherits the error and compounds it down the line. I've watched good work go sideways because someone gauged from a bowed edge. Establish true surfaces first, mark them, and treat them as your single source of truth for the whole part.
In practice
Pick the flattest face and straightest edge, true them if needed, and mark them with a pencil squiggle as your reference face and edge. Always register your square, marking gauge, and tape against those same two surfaces, never a random side. Check the combination square itself against a known straightedge now and then, since a dropped square can quietly go out of true. Once you trust the reference, run every measurement and mark on the part from it consistently.
When it doesn't apply
Rough construction lumber and quick shop jigs rarely need fussed-over references; eyeball it and move on. The discipline matters for furniture and joinery where parts must meet cleanly. And once a board is milled four-square, any face can serve, though marking a consistent reference still prevents confusion.