Seam allowance

The seam line is where the garment actually is. The cutting line is the seam line plus allowance, and it is the only one of the two that gets cut, which is why an allowance applied inconsistently changes the finished measurements without anyone noticing until it is sewn.

Different seams take different allowances: one centimetre on a side seam, more on something that may be let out, less on a curve where a wide allowance will not lie flat, and a specific amount on anything going through an overlocker or a binder.

When a pattern is graded, the allowance has to hold its width around the new outline — including through the concave curves — rather than being scaled with everything else.