Bias tape or bias binding is a narrow strip of fabric, cut on the bias (UK cross-grain). The strip's fibers, being at 45 degrees to the length of the strip, makes it stretchier as well as more fluid and more drapeable compared to a strip that is cut on grain. Many strips can be pieced together into a long "tape". The tape's width varies from about 1/2" to about 3" depending on applications. Bias tape is used in making piping, binding seams, finishing raw edges, etc.
Commercially available bias tape is available as a simple bias tape, single-fold bias tape, and double-fold bias tape. Single-fold bias tape is bias tape that is folded in half lengtwise on its center line, wrong side together, and pressed. Double-fold bias tape is single-fold bias tape where each half is again folded in half lengthwise, wrong sides together, and pressed.
See also trim (sewing).