The properties of heteroanionic materials depend on their chemical composition; specifically the identities and relative stoichiometries of their constituent anions, and on their structure, particularly the arrangement of these anions within their host crystal structure. While some heteroanionic materials are crystallographically ordered, with their constituent anion species arranged in a regular, repeating pattern, others are crystallographically disordered, with their anion species randomly distributed across crystallographically equivalent sites. In these anion-disordered systems, at long range, the site occupations of these anion species are uncorrelated. At short range, however, these different anion species often exhibit short-range ordering, characterised by one or more local configurations of anions appearing more frequently than in a fully uncorrelated (maximum entropy) anion distribution.