Usually weaving uses shedding devices. These devices pull some of the warp threads to each side, so that a shed is formed between them, and the weft is passed through the shed. There are a variety of methods for forming the shed. At least two sheds must be formed, the shed and the countershed. Two sheds is enough for tabby weave; more complex weaves, such as twill weaves, satin weaves, diaper weaves, and figured (picture-forming) weaves, require more sheds. Shed-rod A shed-rod is simply a stick woven through the warp threads. When pulled perpendicular to the threads, it creates a shed. To create the counter-shed, a heddle-bar is usually used. Heddle-bar A heddle-bar is simply a stick placed across the warp and tied to individual warp threads. When it is lifted, it pulls the warp threads it is tied to out of position, creating a shed. A warp-weighted loom (see diagram) typically uses a heddle-bar. It has two upright posts (C); they support a horizontal beam (D), which is cylindrical so that the finished cloth can be rolled around it, allowing the loom to be used to weave a piece of cloth taller than the loom, and preserving an ergonomic working height. The warp threads (F, and A and B) hang from the beam and rest against the shed rod (E). The heddle-bar (G) is tied to some of the warp threads (A, but not B), usign loops of string called leashes (H). So when the heddle rod is pulled out and placed in the forked sticks protruding from the posts (not lettered, no technical term given in citation), the shed (1) is replaced by the counter-shed (2). By passing the weft through the shed and the counter-shed, alternately, cloth is woven. Heddle-rods are used on modern tapestry looms. Tablet weaving Tablet weaving uses cards punched with holes. The warp threads pass through the holes, and the cards are twisted and shifted to created varied sheds. This shedding technique is used for narrow work. It is also used to finish edges, weaving decorative selvage bands instead of hemming. Rotating-hook heddles There are heddles made of flip-flopping rotating hooks, which raise and lower the warp, creating sheds. The hooks, when vertical, have the weft threads looped around them horizontally. If the hooks are flopped over on side or another, the loop of weft twists, raising one or the other side of