The development of new inorganic semiconductors has often been driven by charge-neutral elemental substitutions in already-known semiconductors. Even though these substitutions could be applied on either the cation sites or on the anion sites, history has given a strong preference to the cations, generally leading to the development of multi-cation semiconductors. A well-known example is the substitution starting from CdS to CuInS 2 and further to Cu 2 ZnSnS 4 . The PV field is full of multi-cation materials, such as the inorganic halide perovskite CsPbI 3 and various other ternary absorbers such as ZnSnN 2 and AgBiS 2 . Well-established semiconductors such as InAs x P 1-x and CuIn 1-x Ga x S y Se 2-y are often synthesized with more than one anion. However, these materials are not genuine multi-anion compounds but rather anion solid solutions, in which both anions occupy the same crystallographic sites with different fractional occupancies. In these cases of anion solid solutions, the properties of, say, InAs x P 1-x can be predicted with reasonable accuracy from those of the InAs and InP binaries using Vegard's law. Solid solutions are useful for finetuning the properties of their constituent binaries, such as lattice parameters and band gaps. However, unique or unexpected properties are rare. This behavior is in stark contrast with the genuine multi-cation semiconductors mentioned in the previous paragraph. These are unique compounds where each cation occupies a specific crystallographic site. Hence the properties of CsPbI 3 are unique, instead of simply being a weighted average of the properties of CsI and PbI 2. With the additional degree of freedom given by the extra cation, CsPbI 3 turns out to be an excellent PV absorber, even though CsI and PbI 2 are not.