The traditional name for a tetrahedrally coordinated carbon atom bonded to four different entities, two and only two of which have the same @[email protected] but opposite @[email protected] The r/s descriptors of [email protected]@ carbon atoms are invariant on reflection in a mirror (i.e. r remains r, and s remains s), but are reversed by the exchange of any two entities (i.e. r becomes s, and s becomes r). An example is C-3 of ribaric (C-3 is r) or xylaric acid (C-3 is s) or hyoscyamine (C-3 is r). The hyphen in [email protected]@ may be omitted.