https://doi.org/10.1351/goldbook.C01088

Conformations referring to torsion angles θ (A, B, C, D), where A, B, C, D are main-chain atoms, can be described as: *cis* or synperiplanar (C); gauche or synclinal (G); anticlinal (A); and *trans* or antiperiplanar (T), corresponding to torsion angles within ± 30° of, respectively, 0°, ± 120° and ± 180°. The letters shown in parentheses (upper case C, G, A, T) are the recommended abbreviations. The symbols G+, G- (or A+, G-, for example) refer to torsion angles of similar type but opposite known sign, i.e. ~ +60°, ~ -60° (or ~ +120°, -120°). The notation G, G(bar) ; A, A(bar) (and T, T(bar) ; C, C(bar) - whenever the torsion angles are not exactly equal to 180° and 0°, respectively) is reserved for the designation of enantiomorph conformations, i.e. conformations of opposite but unspecified sign. Where necessary, a deviation from the proper value of the torsion angle can be indicated by the sign (~), as in the following examples: G(~); G^+ (~); G^- (~).*Source: *

Purple Book, 1^{st} ed., p. 41 (http://old.iupac.org/publications/books/author/metanomski.html)

Purple Book, 1