A ruler on an isometric drawing
Isometric drawing has one scale — 0.8165 — and every account of it stops there. But that number is about three directions and a drawing has infinitely many, so a length measured off the paper and divided by 0.8165 comes back anywhere between √(1/2) and √(3/2) of the truth: 29.3% short to 22.5% long, with nothing in the picture to say which.
rung 4 · parallel