‘Yarn' is the collective name for threads which have been spun for knitting. These can consist of wool, cotton, nylon, silk, hair, flax, hemp, metals or numerous man made fibres, all of which can be used alone or in various combinations.
These strands are known to the spinners as ‘counts' and they, in turn, make up a ply, 2, 3, 4 or more, which make up the yarn.
A ply can be any number of strands or counts and does not necessarily refer to the thickness of the yarn. It is for this reason that it is not advisable to substitute one yarn for another when working from a pattern. A 2-ply can be thicker than a 3-ply, as is the case with the Shetland or other homespun yarns, and it is most important to use the particular yarn recommended for a design.