Bias tape

Bias tape, also known as bias binding, is made from strips of fabric which are used as edging in a variety of sewing projects, including the making of clothing and quilting work. Bias tape is, as the name implies, a long strip of fabric cut on the bias. The bias of a fabric is also known as the cross-grain, and refers to a cut made at a 45° angle to the threads of the fabric. Basic fabrics are made by weaving threads together at 90° angles. Thus by cutting along a 45° angle, the cut is made directly across, rather than along, the lines of the woven threads.

Bias makes an appropriate edging in sewing by adding flexibility and support to the edge of the fabric, and preventing fraying. Cutting along the bias allows the woven threads to share the tension of pulling or stretching fabrics, rather than letting either the horizontal or vertical threads absorb all the tension on their own.
Due to its flexibility and finished look, bias tape is often used in place of a hem at the edge of a piece of fabric. Bias tape can be used directly on raw edges of fabric, or can be used to make other types of edging, including piping and binding seams. Bias tape is commonly used in armholes, necklines, straps, or ties in both clothing and bags.
Bias tape is available for purchase in sewing and craft stores, but it is only offered in a select variety of colors. For this reason, many seamstresses and craftspeople elect to make their own bias tape. This can be easily accomplished by cutting the desired fabric along the cross-grain to make a long strip. The strip of fabric can be folded and either pressed, or sewn to finish the edge of the bias tape itself, to make it appear more attractive.
Simple bias tape is little more than a strip of bias-cut fabric, whereas single-fold and double fold bias tapes are more complicated, and can perhaps provide more structural support to a garment. Single-fold bias tape is made by folding the strip of fabric in half lengthwise, with wrong sides together, and pressing the fabric to form a distinct edge. Double-fold bias tape is made simply by folding a single-folded bias tape in half again.
It is important to cut a fabric strip wide enough that it will still serve as an edging once it has been folded to make the bias tape. The length is also important to consider when cutting the fabric strip. If the strip is not long enough, two strips may have to be sewn together at the ends, perhaps compromising both the strength and the appearance of the bias tape. When executed correctly, the use of bias tape can lend both support and decorative detail to a garment, and prevent the warping of a curved edge of fabric that might occur with a straight grain strip of fabric, or a simple hem.