Grading and Trimming Seam Allowances for Clean Curves

Wherever several layers of fabric stack up at a seam, especially an enclosed one such as a collar, cuff or faced edge, the combined thickness creates a ridge that shows through to the right side and prevents a crisp edge. Reducing that thickness is the goal of all these techniques.

Enclosed seams, where the allowance is trapped inside, are the prime candidates, because the bulk has nowhere to go and will press into a visible lump unless it is reduced first.

Grading means trimming each layer of a seam allowance to a different width so they step down rather than ending in one thick edge. The layer closest to the outside is left widest, with each successive layer trimmed narrower, so the bulk tapers and does not print a ridge onto the right side.

It is the essential treatment for collars, lapels, cuffs, waistbands and any faced edge. Without grading, an enclosed seam pressed flat leaves a hard line visible from outside.

At a corner such as a collar point or a square neckline, the allowance bunches when turned, producing a blunt, lumpy point. Trimming the allowance close and cutting across the very tip removes that excess so the point turns out sharp.

On sharp points, also trim the allowances on each side of the tip at an angle to taper them further. The aim is to leave just enough allowance to hold the seam without crowding the point.

A concave, or inward, curve such as a neckline cannot spread to lie flat because its allowance is shorter than the seam it must follow. Clipping small snips into the allowance, up to but not through the stitching, lets the allowance open out and the curve lie smooth.

Space the clips closely enough that the curve relaxes fully, but not so deep that they reach the stitch line, which would weaken or open the seam.

A convex, or outward, curve such as a collar's outer edge has the opposite problem: its allowance is longer than the seam, so it bunches and overlaps when turned. Cutting small wedge-shaped notches out of the allowance removes the excess so the curve lies flat without overlapping folds.

As with clipping, the notches stop short of the stitching. Together, clipping and notching let any curved seam turn cleanly into a smooth line.

• Concave curve (inward): clip with single snips so the allowance spreads.
• Convex curve (outward): notch out wedges so the allowance contracts.
• Corners and points: trim close and across the tip to remove bulk.
• Enclosed seams: grade the layers so the bulk steps down.

Every cut into a seam allowance is a small compromise of strength, so the discipline is to cut only as much as needed. Never clip or notch through the stitching, and on fabrics that fray badly, finish or stay the edge or keep clips shallower and more frequent.

Reinforcing a tightly curved or heavily clipped seam with a second line of stitching just inside the first gives security where the allowance is most reduced. Balance is the rule: enough cutting to lie flat, never so much that the seam fails.