A chain-adapted symmetry-mode analysis is presented for the symmetry change R 3 m > C12/c1 in ferroelastic lead phosphate. The primary and the secondary modes are derived for all relevant Wyckoff positions. The splitting schemes of the orbits and the patterns of the displacive modes are compatible with experimental diffraction studies performed in lead phosphate type crystals. The appearance of diffuse monoclinic X-ray diffraction signals above the ferroelastic transition point of lead phosphate is modelled using simple real structure simulations.