Definition
Bias is any direction diagonal to the grain of a woven fabric; true bias runs at 45 degrees to the selvedge and gives the most stretch and drape.
Woven threads barely stretch along their length or width, but the square grid they form can deform diagonally. Cut at 45 degrees, the true bias, a woven fabric stretches noticeably, molds over curves and falls in soft, clinging folds. In British English, cutting on the bias is also called cutting on the cross.
Sewists use that in three ways:
- Bias-cut garments. Skirts and dresses cut on the bias skim the body and swing, like a slip dress. They need more fabric, and their hems drop unevenly: hang the garment for 24 hours before leveling and hemming.
- Bias strips. Strips cut on the bias bend smoothly around necklines and armholes, which is why bias binding is cut that way.
- Edges to protect. Necklines, armholes and the waist of a circle skirt are partly on the bias and stretch while you handle them. A line of staystitching right after cutting keeps them to size.
To find the true bias, fold the fabric so a crosswise cut edge lies along the selvedge; the fold is the 45-degree line.
Handle bias edges with as little pulling as possible, pin or baste before sewing so the layers do not grow, and press by lifting the iron rather than sliding it.