Stitch a curved seam, turn it right side out, and one of two things happens. Either the allowance pulls tight and puckers the seam, or it bunches up and creates lumps. Both are geometry, and once you can name which case you are looking at, the fix is obvious.
The Geometry Behind Clipping and Notching
On a concave curve, such as a neckline or the underarm of an armhole, the seam allowance sits on the outside of the curve when turned. The stitching line has a shorter path than the raw edge needs, so the allowance is too short and pulls. The remedy is to clip: make straight snips perpendicular to the stitching line, letting the allowance spread open.
On a convex curve, such as the outer edge of a collar or a pocket flap, the allowance sits on the inside of the curve when turned. Now the allowance has too much length and must compress into a shorter path, which creates bulk. The remedy is to notch: cut small wedges or V shapes out of the allowance so the surplus is removed rather than crushed.
The shorthand worth memorising is that concave curves need room, so you clip; convex curves have too much, so you notch. A seam that is concave on one piece is convex on the other, so you may do both on the same seam if it changes direction.
How Far to Cut
Stop 2 to 3 mm short of the stitching line. Cutting to the stitching or past it leaves a weak point where the fabric can tear away from the seam under load, and the damage is often invisible until the garment has been worn a few times. Space the cuts according to the tightness of the curve: a gentle curve may need clips every 2 cm, a tight neckline every 6 to 8 mm.
Fabric structure changes how much you can get away with. Loosely woven cloth and any allowance that will bear tension deserve fewer, shallower clips, because yarns that are cut short have nothing anchoring them. Work on woven fabric cut on the bias shows that when a woven sample is cut at an angle to the yarn directions, no yarn is held at both ends and the structure relies on friction at the interlacing points to carry load. Clipping a curve creates the same condition locally, which is exactly why it must be done sparingly.
Curves and Corners at the Machine
Shorten the stitch length to about 1.8 to 2 mm on curves and around corners. Shorter stitches follow a curve more accurately and, at a corner, put more anchoring stitches into the point so the turned corner holds its shape.
To pivot at a corner properly, stop with the needle down, lift the presser foot, rotate the fabric, and lower the foot before continuing. Stopping the needle in the wrong place is the usual reason corners come out rounded. On a sharp corner such as a collar point, take one or two stitches diagonally across the corner instead of a single hard pivot. That short diagonal gives the allowance somewhere to go and produces a crisper point than a true right angle ever does.
Trim corners before turning. Cut across the point close to the stitching, then taper each side allowance away from the point. On a very sharp collar point, trim a second time at a shallower angle. Use a point turner rather than scissor tips, which will push straight through the corner.
Grading and Understitching
Grading means trimming the layers of a seam allowance to different widths so they end in a staircase rather than a cliff. The layer that will sit against the outside of the garment stays widest. Without grading, a curved faced edge on medium or heavy fabric shows a visible ridge from the right side.
Understitching is the finishing move that keeps facings and linings where they belong. After clipping, notching and grading, press the seam allowance towards the facing, then stitch through the facing and both allowances about 2 mm from the seam. The facing now rolls to the inside on its own and stays there through wear and laundering, with no topstitching visible on the outside. It takes a minute and it is the difference between a neckline that behaves and one that flips.
All of this rests on stitch quality. Since stitch length and needle characteristics measurably influence how much a seam deforms and how much force it withstands, the settings you choose for a tight curve are doing structural work, not just cosmetic work.