When the rings of a steroid are represented as projections onto a plane in the conventional manner, an atom or group is called if it projects below the plane and if it projects above the plane. This '/-system' has been extended to other cyclic natural products and related products (see, for instance, Section E, Stereochemistry and Stereoparents, pp. 192I-212I, in 'Chemical Substance Index Names', Appendix IV, in the 1992 Index Guide, Chemical Abstracts Services, Columbus, Ohio 1992).

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