ISO 2

{{Short description|ISO standard}}

{{For|two-letter country codes|ISO 3166-1 alpha-2}}

{{Use British English Oxford spelling|date=December 2011}}

File:Yarn twist S-Left Z-Right.png

ISO 2 is an international standard for direction of twist designation for yarns, complex yarns, slivers, slubbings, rovings, cordage, and related products.{{Citation|title=ISO 2:1973 Textiles — Designation of the direction of twist in yarns and related products|author=International Organization for Standardization|author-link=International Organization for Standardization|url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=3563|year=1973|access-date=2013-08-04}}

The standard uses capital letters S and Z to indicate the direction of twist,{{Citation|title=Terminology of man-made fibres|author=The International Bureau For The Standardisation of Man-Made Fibres (BISFA)|page=49|url=http://www.bisfa.org/Portals/BISFA/Terminology/BISFA%20Terminology2009%20%28final%20version%29.pdf|year=2009|access-date=2013-08-04|archive-url=https://web.archive.org/web/20151123184955/http://www.bisfa.org/Portals/BISFA/Terminology/BISFA%20Terminology2009%20(final%20version).pdf|archive-date=2015-11-23|url-status=dead}} 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.{{Cite book|last=Himmelfarb|first=David|title=The Technology of Cordage Fibres and Rope|location=London|publisher=Leonard Hill|year=1957|page=116}}