Woolen, semi-worsted and worsted refer to the arrangement of fibers in a single ply yarn.
The fibers in woolen yarn are as randomly arranged as possible, to increase the number of air spaces. Short fiber is carded and made into rolags, which are spun from the end by drafting against the advancing twist. Fibers up to about 8cm in length can successfully be spun by this method, called "unsupported long draw" or "double drafting." Woolen yarns are light and fuzzy and generally have less twist to preserve the lofty character. This also means they are less stable. Wool yarns are more likely to felt and work best when the finished fabric is fulled. The unsupported long draw is a one hand spinning technique, which gives less control over yarn diameter. Historically, industrial woolen spinning was more concerned with lofty weft and good filling coverage than exact control of yarn size because the fabrics were fulled. A quill of handspun woolen yarn was sometimes used as a bobbin for weaving, as the softest yarns cannot withstand skeining.
The fibers in worsted yarn are arranged parallel, with as little air as possible. Long fibers reduce the number of fiber ends and produce a smooth surface. Worsted yarns always use a combed preparation, which removes shorter fibers while it aligns the longer ones. The yarn is drafted without twist by pulling fibers from the top and then allowing the twist to enter while smoothing the yarn. Worsted yarns are strong, smooth and heavy. Wool yarns are less prone to felt. The two hand drafting technique gives fine control over yarn diameter. Worsted yarns are much more suited for warp than woolen. Traditional wool combing for worsted is an involved procedure and one of the last processes successfully mechanized.
Woolen and worsted are at opposite extremes of fiber arrangement. Between them is semi-worsted, which is everything that is not pure woolen or pure worsted. This system of yarn classification has its origins in the pre-industrial English worsted and woolen industries, which had strict rules about the required methods for each. Even now, some spinners have strong opinions concerning equipment and procedures, particularly for worsted. Also, semi-worsted refers to spinning a woolen-style preparation of carded fiber with a worsted-style spinning technique. Similarly, semi-woolen is worsted-style combed fiber with a woolen-style technique. Many yarns strictly categorized as "semi-worsted" are nearly indistinguishable from worsted.
Maximum 12 points
Both examiners gave me full points, so presumably nobody had a problem with it. It took a long time to come up with something that both addressed the range of at times hotly contested opinions on the subject and still could be described as "brief." The use of the word "true" to describe any method of doing something always makes me nervous.