Notch

Notches are the little marks printed on the edge of a pattern piece - usually shown as diamonds, triangles, or short perpendicular lines - that get transferred as actual small cuts into the fabric's seam allowance once you cut the piece out. Their entire job is matching: notch 1 on the sleeve lines up with notch 1 on the armhole, notch 2 with notch 2, and so on, so two curved or eased pieces come together in the right place and orientation rather than by guesswork.

They also solve a second, quieter problem: telling front from back on pieces that otherwise look nearly identical. A single notch usually marks the front of a garment, a double notch the back - a small convention, but one that saves a lot of confusion once you're holding two mirror-image pieces that look the same from across the room.

I cut notches as small, single snips inward - no more than 1/8", never so deep they eat into the actual seamline. A notch cut too deep weakens the seam right where it needs to hold, which defeats the entire point of a marking system meant to help construction, not compromise it.