In dressmaking, grading refers to two separate elements: Grading in pattern-drafting refers to generating various garment sizes. The act of altering a pattern to a larger or a shorter size is referred to as grading. In the sewing process, grading refers to trimming one of the two seam allowance layers to eliminate bulk in the finished fabric edge or seam. The latter type of grading is shown below:
To grade a fabric edge, all you have to do is cut one of the seam allowance layers down the middle, along the entire edge. Doing so will thin-out the seam allowance layers, making them less noticeable along the finished edge.
Grading is an effective tool in eliminating bulk and unnecessary thickness within finished fabric edges and seam allowance layers, It is often used when finishing garment edges with facings, bias strips, or lining. In these instances, the seam allowance layer that touches the facing/lining layer in finished position should be the one trimmed- The longer seam allowance layer should be touching the garment layer.
Grading is also be used with some seam finishing techniques like a self-bound, welt or flat felled seam. With these seam finishes ,one of the seam allowance layers has to be trimmed allowing the wider, untrimmed layer to encase it for a clean finish.