In stereochemistry, a torsion angle is defined as a particular example of a dihedral angle, describing the geometric relation of two parts of a molecule joined by a chemical bond. Every set of three not-colinear atoms of a molecule defines a plane. When two such planes intersect (i.e., a set of four consecutively-bonded atoms), the angle between them is a dihedral angle. Dihedral angles are used to specify the molecular conformation. Stereochemical arrangements corresponding to angles between 0° and ±90° are called "syn" (s), those corresponding to angles between ±90° and 180° "anti" (a). Similarly, arrangements corresponding to angles between 30° and 150° or between −30° and −150° are called "clinal" (c) and those between 0° and ±30° or ±150° and 180° are called "periplanar" (p).