In weaving cloth, the warp is the set of lengthwise yarns that are held in tension on a frame or loom. The yarn that is inserted over-and-under the warp threads is called the weft, woof, or filler. Each individual warp thread in a fabric is called a warp end or end. Warp means "that which is thrown across" (Old English wearp, from weorpan, to throw, cf. German werfen, Dutch werpen).
Because the warp is held under extreme tension during the entire process of weaving, warp yarn must be strong. Yarn for warp ends is usually spun and plied fibre. Traditional fibres for warping are wool, linen and silk. With the improvements in spinning technology during the Industrial Revolution, it became possible to make cotton yarn of sufficient strength to be used as the warp in mechanized weaving. Later, artificial or man-made fibres such as nylon or rayon were employed.