be changed. Such limitations of color placement create weavings defined by length-wise stripes and vertical designs. Many South American cultures, including the ancient Incas and Aymaras, employed backstrap weaving, which uses the weight of the weaver's body to control the tension of the loom. Weft Because the weft does not have to be stretched on a loom the way the warp is, it can generally be less strong. It is usually made of spun fibre, originally wool, flax and cotton, today often of synthetic fibre such as nylon or rayon. The weft is threaded through the warp using a "shuttle", air jets or "rapier grippers". Handlooms were the original weaver's tool, with the shuttle being threaded through alternately raised warps by hand. As metaphor The expression "warp and weft" (also "warp and woof" and "woof and warp") is used metaphorically the way "fabric" is; e.g., "the warp and woof of a student's life" equates to "the fabric of a student's life". Warp and weft are sometimes used even more generally in literature to describe the basic dichotomy of the world we live in, as in, up/down, in/out, black/white, Sun/Moon, yin/yang, etc. The expression is also used similarly for the underlying structure upon which something is built. The terms "warp" and "woof" are also found in some English translations of the Bible in the discussion of mildews found in cloth materials in Leviticus 13:48-59. See also Knot density Pile (textile) Warp knitting Notes References Weaving