It is made 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. Examples of twill fabric are chino, drill, denim, gabardine, tweed and serge.
In a twill weave, each weft or filling yarn floats across two or more warp yarns in a progression of interlacings by one 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 design 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.
Twill fabrics technically have a front and a back side, unlike plain weave, where the two sides are the same. The front side of the twill is the technical face and the back is called 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, and most often used as the fashion side of the fabric. This side is usually the side visible during weaving. If there are warp floats on the technical face (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 (where a design is printed on the cloth) are much less common than printed plain weaves. When twills are printed, they are most likely to be lightweight fabrics. Soil shows less on the uneven surface of twills than it does on smooth surfaces, such as plain weaves. Thus, twills are often used for sturdy work clothing or durable upholstery because soils and stains are less noticeable on this fabric. Denim, for example, is a twill.
The fewer interlacings in twills allow the yarns to move more freely, and thus they are softer and more pliable, and drape better. Twills also recover better from wrinkles than plain-weave fabrics. 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, serge, twill flannel, sharkskin, herringbone, and houndstooth. Warp-faced twills include lining twill, denim, jean, drill, covert, chino, gabardine, cavalry twill, and fancy twill.