In a chain of atoms A-B-C-D, the @D01730@ between the plane containing the atoms A,B,C and that containing B,C,D. In a @N04134@ the torsion @A00346@ is the @A00346@ (having an absolute value between 0° and 180°) between bonds to two specified (@F02356@) groups, one from the atom nearer (proximal) to the observer and the other from the further (distal) atom. The torsion @A00346@ between groups A and D is then considered to be positive if the bond A-B is rotated in a clockwise direction through less than 180° in order that it may @E01884@ the bond C-D: a negative torsion @A00346@ requires rotation in the opposite sense. Stereochemical arrangements corresponding to torsion angles between 0° and ±90° are called @S06216@ (s), those corresponding to torsion angles between ±90° and 180° @A00381@ (a). Similarly, arrangements corresponding to torsion 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). The two types of terms can be combined so as to define four ranges of torsion @A00346@; 0° to 30° synperiplanar (sp); 30° to 90° and -30° to -90° synclinal (sc); 90° to 150°, and -90° to -150° anticlinal (ac); ±150° to 180° antiperiplanar (ap). The synperiplanar @C01258@ is also known as the @S06216@- or cis-@C01258@; antiperiplanar as @A00381@ or @C01092@ and synclinal as @G02593@ or @S05709@. For @M03667@ usage the symbols \(\text{T}\), \(\text{C}\), \(\text{G}^{+}\), \(\text{G}^{-}\), \(\text{A}^{+}\) and \(\text{A}^{-}\) are recommended (ap, sp, +sc, -sc, +ac and -ac respectively).