ISO 2 is an international standard for direction of twist designation for yarns, complex yarns, slivers, slubbings, rovings, cordage, and related products. The standard uses capital letters S and Z to indicate the direction of twist (S indicates a clockwise twist and Z a conter-clockwise twist). The convention of using these two letters to unambiguously designate twist direction was already used in the cordage industry by 1957.
- International Organization for Standardization (1973), ISO 2:1973 Textiles — Designation of the direction of twist in yarns and related products
- The International Bureau For The Standardisation of Man-Made Fibres (BISFA) (2006), Terminology of man-made fibres, p. 46
- Himmelfarb, David (1957). The Technology of Cordage Fibres and Rope. London: Leonard Hill. p. 116.
- ISO 2:1973 Textiles — Designation of the direction of twist in yarns and related products
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