What does cut on the fold mean?

It means the marked edge is laid against the fold of the fabric and never cut, so the piece opens out to twice its width as one symmetrical shape.

Why patterns do it. A front bodice, a back skirt or a facing is symmetrical about the centre. Drawing only half the piece halves the paper, and cutting on the fold guarantees the two halves match exactly — far more reliably than cutting two mirror pieces and joining them.

How to recognise it. The edge is marked with a bracket, a bent arrow pointing to the fold, or the words “place on fold”. That line has no seam allowance, because it is not a seam.

Doing it correctly. Fold the fabric along the grain, parallel to the selvedge, and press the fold. Lay the marked edge exactly on the folded edge — not a millimetre inside it, which narrows the finished piece by twice that amount across the body. Weight or pin the fold edge first, then the rest of the piece.

The usual mistake is cutting along the fold edge as well, which produces two half-pieces and ruins the cut. Fold the paper allowance under, or clip the pattern to the fold, so there is nothing to cut into.

When you cannot fold. On a directional print, a napped fabric or an asymmetric adjustment, trace the mirror image of the piece first and cut it flat as a full piece. That is also the answer when the fabric is not wide enough to fold.