To trim a seam allowance, cut it to about half its width unless otherwise instructed. For light to medium weight fabrics, clip both sides of the seam allowance at once. For heavyweight fabrics, clip each side of the seam allowance separately.
How to Grade a seam allowance
Grading is when the layers of fabric in a seam allowance are trimmed to different widths. It can help reduce bulk when a seam allowance consists of many layers of fabric or if the fabric is very bulky. It can prevent a noticeable ridge or bump from showing on the outside once the item is turned.
To grade a seam allowance, trim the first layer of fabric to about 1/8”. Then trim the next layer of fabric slightly wider, and so on. Trim the seam allowance closest to the outside of the garment the widest.
How to Clip Curves
How to clip inner curves:
Inner curves need to be clipped so that the seam allowance can spread and lie flat once the item is turned. To clip an inner curved seam, clip into the seam allowance at periodic intervals, ending about 1/8” from the stitching. Clip farther apart for shallower curves, closer together for deeper curves.
How to clip outer curves:
Outer curves need to be “notched” to allow the fabric in the seam allowance to compress when the item is turned without producing bulk. To clip an outer curved seam, cut small “V’s” out of the seam allowance at periodic intervals, ending about 1/8” from the stitching. Clip farther apart for shallower curves, closer together for deeper curves.
Remember to clip carefully so you don’t accidentally cut into the line of stitching. And don’ t clip too close to the stitching or it will weaken the seam. If a curved seam will be subject to stress, such as in a princess seam, stagger the clips or notches.
How to Trim Corners
The seam allowances in corners need to be trimmed to reduce bulk in the corner once the item is turned.
To trim the seam allowance at a corner, trim away most of the seam allowance at the tip. Then taper the seam allowances on the sides toward the point of the corner.
If you’re working with a fabric that frays easily, don’t trim too close to the stitching. You may even want to add a row of reinforcement stitching or use a very fine stitch at the corner.
Seam allowances sometimes need to be trimmed or clipped. This is to reduce bulk so that the seam allowance lies flat and smooth once the item is turned. Your pattern instructions should tell you when to trim or clip a seam allowance.
Use a small, 3”-5” pair of scissors to make the task easier.
Trim or clip your seam allowance after you’ve sewn your seam and pressed it (see How to Press).