The standard uses capital letters S and Z to indicate the direction of twist, as suggested by the direction of slant of the central portions of these two letters. The handedness of the twist is the direction of the twists as they progress away from an observer. Thus Z-twist is said to be right-handed, and S-twist to be left-handed. 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, retrieved 2013-08-04
- The International Bureau For The Standardisation of Man-Made Fibres (BISFA) (2009), Terminology of man-made fibres (PDF), p. 49, retrieved 2013-08-04
- Himmelfarb, David (1957). The Technology of Cordage Fibres and Rope. London: Leonard Hill. p. 116.