Definition
Grading seams means trimming the layers of a seam allowance to different widths, so they step down in thickness and leave no visible ridge on enclosed seams.
Where several layers meet in an enclosed seam, such as a collar, a faced neckline, a waistband or a cuff, their raw edges all end at the same line and form a hard ridge that shows through on the right side once pressed. Grading staggers them.
How to grade:
- Sew the seam and press it flat as sewn.
- Leave the allowance that lies closest to the outside of the garment the widest, about 6 mm (1/4 in).
- Trim the other allowance, the one on the facing side, to about 3 mm (1/8 in).
- On thick fabrics with several layers, interfacing included, trim each layer a little narrower than the last.
The widest layer cushions the others, so the step is invisible from the outside. Afterward, clip or notch any curves (see clipping and notching) and understitch the facing.
Do not grade seams that will be pressed open, or seams in fraying fabric that need their full width to be finished.
Grading seams has nothing to do with pattern grading, which means resizing a pattern into a range of sizes. The words are the same; the context tells you which is meant.