Lead phosphate–arsenate Pb3(P1−x As x O4)2 undergoes an improper ferroelastic phase transition from a rhombohedral paraphase $R\bar{3}m$ to a monoclinic ferrophase $C2/c$ leading to distinct twin boundary patterns. On cooling compounds with x larger than 0.8 undergo further transitions to monoclinic low-temperature phases, whereas the composition with x = 0.8 shows order-parameter coupling phenomena. The transformation $R\bar{3}m$ –$C2/c$ was described on the basis of a three-state Potts model and the existence of precursors of monoclinic clusters in the rhombohedral paraphase. The system is one of the best studied improper ferroelastics. Due to its two-mode phonon behaviour the solid solution exhibits multistep temperature- as well as pressure-driven structural transformations with different length and time scales. Relevant investigations and findings of this palmierite-type material have been made by Prof E K H Salje. Some of the most prominent results from x-ray diffraction, optical microscopy and Raman scattering are reviewed, and the potential implications for domain-wall structures and engineering are discussed.