Definition
A swayback adjustment is a pattern alteration that removes excess length at the center back waist, for a lower back that curves in more than the pattern assumes.
A pronounced inward curve of the lower back shortens the distance from the back neck to the waist at the center, while the sides stay the same. Garments drafted for a straighter back then have too much length there. It shows as horizontal folds pooling above the waist on tops and dresses, or below the waistband on skirts and trousers.
The fix removes a wedge of length at center back, tapering to nothing at the side seam:
- On a toile, pinch out the horizontal fold at center back and measure its depth, often 1 to 2.5 cm (3/8 to 1 in).
- On the back pattern piece, draw a line from center back at that level to the side seam.
- Fold out a wedge of that depth at center back, tapering to zero at the side.
- Redraw center back as a smooth line. If the piece is cut on the fold, straighten it so it can still go on the fold, or add a center back seam.
On skirts and trousers, the equivalent is lowering the center back waist by the same amount, tapering to the side seams, and sometimes reshaping the back crotch curve slightly.
The swayback adjustment is one of the most common back alterations, along with the rounded back adjustment, which adds length at the upper back instead. See fixing common fit issues.