English: Prototypical steroid 5alpha and 5beta stereoisomers, after Moss-IUPAC (1990). The two isomers are defined by a difference in the side of the largely planar ring system that the hydrogen atom at carbon-5 is attached (and a resulting change in the conformation of the steroid A-ring). Shown are the four rings most common to steroids, with standard IUPAC ABCD ring labels (blue), and the standard IUPAC number label for carbon-5 (red); the R at carbon-17 represents possible side chains at that position. The two isomers are drawn in line-angle representation, in the chair-envelope form (rings ABC chairs, ring D envelope), projected onto the plane of the page in the IUPAC-accepted horizontal orientation of the ring system, and with bonds extending above the plane of the page to terminal methyl groups and hydrogen atoms indicated with bold lines, and those extending below the plane of the page as cross-hatched lines. Only the hydrogen atoms at the tertiary carbons are shown explicitly; all remaining hydrogens in the line-angle representation are implied. Alternative representations of these extending bonds are as bold and cross-hatched wedges, respectively (or, for experienced viewers, simple straight lines, where the stereochemical relationships are inferred). Drawing created for "Steroids" article at English Wikipedia by delta0349, by graphical editing of chemical drawing program-generated generic chair-envelope images, using line and text editing features in GraphicConverter9. REFERENCE: G.P. Moss and the Working Party of the IUPAC-IUB Joint Commission on Biochemical Nomenclature, "The Nomenclature of Steroids", hosted at Queen Mary University of London, Section 3S-1.4, incl. Note 4). See [1] and [2], accessed 9 July 2014. Also available from same authors at Pure Appl. Chem. 1989, 61:1783-1822 (see p. 1786f) or R.A. Hill, D.N. Kirk, H.L.J. Makin, H.L.J. & G.M. Murphy, 1991, "Dictionary of Steroids" London:Chapman and Hall, pp. xxx-lix. The Working Party of the IUPAC-IUB ICBN were P. Karlson (chairman), J.R. Bull, K. Engel, J. Fried^†, H.W. Kircher, K.L. Loening, G.P. Moss, G. Popják and M.R. Uskokovic. [Le Prof]