This section is from the "A Complete Dictionary of Dry Goods" book, by George S. Cole. Also available from Amazon: A complete dictionary of dry goods and history of silk, cotton, linen, wool and other fibrous substances,: Including a full explanation of the modern processes ... together with various useful tables.
Twill. [From German twillen, to separate into two parts] An appearance of diagonal lines or ribs produced in textile fabrics by causing the weft threads to pass over two and under one, or over three (or more) warp threads, instead of over one and under the next in regular succession, as in plain weaving. The object of twill weaving is to increase the weight or bulk of a fabric and to ornament it. The disposition of the threads permits the introduction of more material into the fabric, and hence makes it more bulky and closer in construction than in plain weaving. Many different patterns or surfaces can be produced by twilling that is by changing the order of passing the weft over the warp threads; as a satin twill, plain twill, 3 - leaf and 4 - leaf twills and a damask twill. The regularity of the twilled lines is broken in various ways in what is termed fancy twilling; as in herringbone and zig-zag weaving. The effect of each variety of twilling is to form a distinct pattern upon the fabric. [See Weaving, Satin, Damask]