The bond valence model, which is derived from the ionic model, is expressed through a number of rules and equations that determines which acid-base bond structures can exist. Chief among these rules is the bond valence sum rule, which states that the sum of bond valences around an ion is equal to its atomic valence. These rules can be used to understand many of the properties of inorganic structures, such as bond lengths, coordination numbers, their structures and their solution chemistry. The unusual geometries and properties of hydrogen bonds follow naturally from these rules. Because the model describes chemically ideal structures, it allows one to quantify the role of electronic anisotropies and steric strain in observed structures, the latter frequently leading to phase transitions in crystals. In favourable cases the model can be used for structure prediction by constructing the bond network ab initio and then mapping this onto a compatible space group. The model has applications in many fields ranging from earth sciences to biology.