The area between the raw edge of a fabric and sewing line, that allows two pieces of a pattern to be sewn together. The size of the seam allowances depends on the sewing technique, the type of fabric used, as well as the preferences of a sewer or a pattern maker (seam allowances are often already added to the pattern). It’s very important to sew the seam allowances accurately for the pattern pieces to match up.
The most commonly used seam allowance sizes are: 1/4″, 3/8″ and 1/2″ (0.6 cm, 1 cm, 1.3 cm). When sewing made to measure clothes, some seam allowances e.g. on the sides are often cut even bigger (about 3/4″ or 2 cm) which allow the garment to be fitted to the wearer.