Add the Third Number to Stair Layout: Check Every Step Against the Diagonal

The Problem

Stringer layout with a framing square is a repeated operation, and repeated operations accumulate error. A sixteenth of drift per step compounds across a dozen steps into treads that don't land level and a stringer that misses its landing — and nothing in the standard rise-run stepping tells you it's happening until the end.

The Technique

A framer's self-checking layout, built on the third number nobody uses:

1. Start with a template and your rise and run — standard so far.
2. Calculate the diagonal — the straight-line distance between the rise point and the run point (the hypotenuse of each step's triangle). That's the third number in every step, and it's the one that can't lie: if rise and run are both right, the diagonal is exactly this length; if either drifted, it isn't.
3. Mark the diagonal on the template.
4. Set stair gauges on your framing square so the square registers identically for every step, and step off the layout as usual.
5. At each step, the diagonal mark is the proof: the square's rise and run points must hit the diagonal's endpoints. Any creep shows up immediately as a miss — at the step where it happened, not at the bottom of the stringer where it's unfixable.