Bias (fabric)

The bias is the diagonal direction of a woven fabric, running at 45 degrees to the selvedge, and understanding it helps cosplayers cut and drape more accurately.

Every woven fabric has two straight grains — the lengthwise warp threads and the crosswise weft threads — and then there is the bias, the diagonal direction sitting at roughly 45 degrees between them. Along the straight grain, woven fabric is quite stable and resists stretching. Along the bias it behaves very differently, giving noticeably when you pull it and draping in soft, fluid curves rather than holding a rigid shape. This stretch is useful and troublesome in equal measure. Costume pieces cut deliberately on the bias — a flared skirt, a cowl hood, or a draped sash — will hang with a flowing, almost liquid quality that straight-grain cutting cannot easily achieve. On the other hand, if you accidentally place a pattern piece on the bias when it should sit on the straight grain, seams can twist, necklines can sag, and hems can dip unevenly after you hang the garment. Checking your grain line against the selvedge before you cut, and testing the effect on a mock-up first, will save a great deal of frustration later.