In general, weaving involves the interlacing of two sets of yarns at right angles to each other: the warp and the weft. The warp are held taut and in parallel order by a loom. The loom is warped (or dressed) with the warp yarns passing through heddles on two or more harnesses. The warp yarns are moved up or down by the harnesses creating a space called the shed. The weft yarn is wound onto spools called bobbins. The bobbins are placed in a shuttle which carries the weft yarn through the shed. The raising and lowering sequence of warp yarns gives rise to the three possible weave structures: plain weave, twill weave and satin weave.