Many definitions that describe a specific conformer (IUPAC Gold Book) exist, developed by William Klyne and Vladimir Prelog, constituting their Klyne–Prelog system of nomenclature:
* a torsion angle of ±60° is called gauche
* a torsion angle between 0° and ±90° is called syn  (s)
* a torsion angle between ±90° and 180° is called anti (a)
* a torsion angle between 30° and 150° or between –30° and –150° is called clinal
* a torsion angle between 0° and 30° or 150° and 180° is called periplanar (p)
* a torsion angle between 0° to 30° is called synperiplanar or syn- or cis-conformation (sp)
* a torsion angle between 30° to 90° and –30° to –90° is called synclinal or gauche or skew (sc)
* a torsion angle between 90° to 150°, and –90° to –150° is called  anticlinal (ac)
* a torsion angle between ±150° to 180° is called  antiperiplanar or anti or trans (ap).
Torsional strain results from resistance to twisting about a bond.