Seam allowance is the area between the edge and the stitching line on two (or more) pieces of material being stitched together. Seam allowances can range from 1/4 inch wide (6.35 mm) to as much as several inches. Commercial patterns for home sewers have seam allowances ranging from 1/4 inch to 5/8 inch.
Sewing industry seam allowances range from 1/4 inch for curved areas (e.g. neck line, armscye) or hidden seams (e.g. facing seams), to one inch or more for areas that require extra fabric for final fitting to the wearer (e.g. center back).