The Mossbauer single-crystal experiment is discussed with particular reference to the low-symmetry resonant sites. It is demonstrated that ambiguities associated with multiple, symmetry-related resonant sites (that is, sites whose Laue class is lower than that of the host crystal) are resolvable provided the absorber recoilless fractions can be precisely measured. A general computer program, MOSREF, is described that enables simulation of single-crystal spectra and/or refinement of Mossbauer parameters for a ⁵⁷Fe centre in a site of any known symmetry in a host crystal of any known symmetry. The procedures are illustrated for a couple of ideal examples: a triclinic site in a monoclinic crystal and a monoclinic site in a trigonal crystal. Applications to recent experiments with the sheet micas biotite and muscovite are given.