From McCall’s Sewing in Colour
Often when working with commercial patterns they will ask for a seam to be ‘graded’. This term refers to a process which makes the seam thinner, or lighter & reduces the bulk of a garment. It is actually something which will help any Hand Made garment look more professionally finished & make the finished, ironed seams look truly beautiful.
Grading the seam is mainly found when lining or facing a garment however; it is excellent when sewing heavier weight fabrics such as Wool, Canvas, Denim or even some Linens. Overall it is a technique which will stop there being a ridge on the right side of the finished garment when ironed. Because a seam can not be neatened- by zig-zagging, serging or overlocking- after it has been graded it is mainly found in seams which will be hidden in the final garment.
Press the seams in one direction (the pattern notes should advise which direction) to ‘set’. Trim one seam as close to the stitching as is comfortable- the depth will differ with different weight fabrics. Trim the other seam down to about half it’s original depth. Iron again, always in the correct direction, and admire!