The essential feature of a worsted yarn is straightness of fibre, in that the fibres lie parallel to each other. Traditionally, long, fine staple wool was spun to create worsted yarn, but other long fibres are also used today.
Many spinners differentiate between worsted preparation and worsted spinning. Worsted preparation refers to the way the fibre is prepared before spinning, using gilling machines which force the fibre staples to lie parallel to each other. Once these fibres have been made into a top, they are then combed to remove the short fibres. The long fibres are combined in subsequent gilling machines to again make the fibres parallel. This produces overlapping untwisted strands called slivers. Worsted spinning refers to using a worsted technique, which produces a smooth yarn where the fibres lie parallel .
Roving and wool top are often used to spin worsted yarn. Many hand spinners buy their fibre in roving or top form. Top and roving are ropelike in appearance, in that they can be thick and long. While some mills put a slight twist in the rovings they make, it is not enough twist to be a yarn. The fibers in top and rovings all lie parallel to one another along the length, which makes top ideal for spinning worsted yarns.
Worsted-spun yarns, used to create worsted fabric, are spun from wool fibers that have been combed, to ensure that the woollen fibers all run the same direction, butt-end (end that was cut in shearing the sheep) to tip, and remain parallel. A short-draw is used in spinning worsted fibers (as opposed to a long -draw).
In short draw spinning, spun from combed roving, sliver or wool top, the spinners keep their hands very close to each other. The fibers are held fanned out in one hand while the other hand pulls a small number from the mass. The twist is kept between the second hand and the wheel - there is never any twist between the two hands.
Worsteds differ from woolens, in that the natural crimp of the wool fibre is removed in the process of spinning the yarn. In Tropical Worsteds this use of tightly-spun straightened wool combined with a looser weave permits the free flow of air through the fabric.
The term "worsted" is often applied to any yarn spun from fibres three inches in length or longer that have been carded or combed, and spun, not just wool. Acrylic and other yarns can be called "worsted," as much a reference to the weight of the yarn as the production process.
A worsted yarn has a thickness of 12 wraps per inch. Depending on a knitter's personal technique, a worsted yarn generally has a gauge of about 16-20 stitches per 10 centimeters using 5.5mm (US size 9) needles.
Prior to the introduction of automatic machinery there was little difficulty in attaining a straight fibre, as long wool was always used, and the sliver was made up by hand, using combs. However, with the introduction of Richard Arkwright's water frame in 1771, and the later introduction of cap and mule spinning machines, the need for perfectly prepared slivers became apparent, and many manufactories used one or more preparatory "gill-boxes" (combing machines) before the worsting process, to ensure straightness of fibre and distribute the lubricant evenly.