The atomic displacements of many of the atoms in a macromolecular structure can be modelled in terms of group motions described in the harmonic approximation by T, L and S tensors. Relevant groups may be planar side groups of protein chains, units of secondary structure such as α-helices or whole protein domains. For the TLS parameters to be interpreted, they must be related to the axes of inertia of the rigid groups and, in the case of the T and S tensors, must be calculated with respect to the centre of reaction of the rigid group. A program (TLSANL) is described that analyses these 21 TLS rigid-body displacement parameters and their relation with the principal axes of the rigid body, from the output of the segmented anisotropic refinement of a macromolecular structure, as produced by a program such as RESTRAIN [Haneef, Moss, Stanford & Borkakoti (1985). Acta Cryst. A41, 426–433; Driessen, Haneef, Harris, Howlin, Khan & Moss (1989). J. Appl. Cryst. 22, 510–516].