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ISO 2 est un international standard pour la désignation de la direction de torsion de yarns, complex yarns, slivers, slubbings, rovings, cordage, and de produits relatifs.[1] La convention d'utiliser ces deux lettres pour désigner sans ambiguïté la direction de torsion était déjà utilisée dans l'industrie de cordage en 1957.[2]
![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/2/25/Yarn_twist_S-Left_Z-Right.png/169px-Yarn_twist_S-Left_Z-Right.png)
The standard uses capital letters S and Z to indicate the direction of twist,[3] 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.