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Twill is a type of textile weave with a pattern of diagonal parallel ribs (in contrast with a satin and plain weave). This is done by passing the weft thread over one or more warp threads and then under two or more warp threads and so on, with a "step" or offset between rows to create the characteristic diagonal pattern. Because of this structure, twills generally drape well.
In a twill weave, each weft or filling yarn floats across the warp yarns in a progression of interlacings to the right or left, forming a distinct diagonal line. This diagonal line is also known as a wale. A float is the portion of a yarn that crosses over two or more yarns from the opposite direction.
A twill weave requires three or more harnesses, depending on its complexity. A twill weave is the second most basic weave that can be made on a fairly simple loom.
Twill weave is often designated as a fraction—such as 2⁄1—in which the numerator indicates the number of harnesses that are raised (and, thus, threads crossed—in this example, two), and the denominator indicates the number of harnesses that are lowered when a filling yarn is inserted (in this example, one). The fraction 2⁄1 would be read as "two up, one down". The minimum number of harnesses needed to produce a twill can be determined by totaling the numbers in the fraction. For the example described, the number of harnesses is three. (The fraction for plain weave is 1⁄1.)
Twill fabrics technically have a front and a back side, unlike plain weave, whose two sides are the same. The front side of the twill is the technical face; the back is called the technical back. The technical face side of a twill weave fabric is the side with the most pronounced wale; it is usually more durable, more attractive, most often used as the fashion side of the fabric, and the side visible during weaving. If there are warp floats on the technical face (i.e., if the warp crosses over two or more wefts), there will be filling floats (the weft will cross over two or more warps) on the technical back. If the twill wale goes up to the right on one side, it will go up to the left on the other side. Twill fabrics have no up and down as they are woven.
Sheer fabrics are seldom made with a twill weave. Because a twill surface has interesting texture and design, printed twills (on which a design is printed on the cloth) are much less common than printed plain weaves. When twills are printed, they are typically done so on lightweight fabrics.
Soil and stains are less noticeable on the uneven surface of twills than on smooth surfaces, such as plain weaves. Thus, twills are often used for sturdy work clothing or durable upholstery. Denim, for example, is a twill.
The fewer interlacings in twills allow the yarns to move more freely, and thus they are softer, more pliable, and drape better than plain-weave textiles. Twills also recover from wrinkles better than plain-weave fabrics do. When there are fewer interlacings, yarns can be packed closer together to produce high-count fabrics. In twills and higher counts, the fabric is more durable and air- and water-resistant.
There are even-sided twills and warp-faced twills. Even-sided twills include foulard or surah, herringbone, houndstooth, serge, sharkskin, and twill flannel. Warp-faced twills include cavalry twill, chino, covert, denim, drill, fancy twill, gabardine, and lining twill.
A more detailed discription of Twill can be found in PPD 2.29 or soon to be released PPD 2.40.