A sway back adjustment removes excess length at the centre back, where fabric pools in horizontal folds just above the seat, while leaving the side seams alone.
Recognise it first. The sign is a horizontal fold or a small pouch of fabric across the back waist that disappears if you pinch out a fold at the centre back. It is a length problem at one point, not a width problem, and taking in the side seams makes it worse.
On a bodice or a dress. Draw a horizontal line across the back piece at the level of the fullness, from the centre back to the side seam. Slash along it from the centre back, stopping just short of the side seam so the paper stays hinged. Overlap the cut edges at the centre back by the amount you pinched — commonly 10 to 20 mm — and tape. The side seam length is unchanged.
Then true the centre back, which now has a step in it. Redraw it as a straight or smoothly curved line, and check that it still meets the waist seam at a right angle.
On trousers or a skirt, the same principle applies at the back waist, and the waistband seam is redrawn to match.
Check the darts. After overlapping, the back darts may be shorter than intended. Redraw them from the new waistline.
Then walk the seams against the front piece and the waistband, and test on a toile before cutting the real fabric.
Questions people actually ask
How much should I remove?
Pin out the fold on a toile and measure it. Guessing produces a back that is now too short, which is harder to see and harder to fix.
Is it the same as a short back adjustment?
No. A short back removes length across the whole width; a sway back removes it at the centre only, tapering to nothing at the side seam.
Last reviewed 2026-08-11