Although there is a large number of simple known ABX perovskites, this number can be greatly expanded if the A and B sites are increasingly doubled / complex AA’BB’X. Ordered double perovskites are usually denoted as ABB’O where disordered are denoted as A(BB’)O. In ordered perovskites, three different types of ordering are possible: rock-salt, layered, and columnar. The most common ordering is rock-salt followed by the much more uncommon disordered and very distant columnar and layered. The formation of rock-salt superstructures is dependent on the B-site cation ordering. Octahedral tilting can occur in double perovskites, however Jahn–Teller distortions and alternative modes alter the B–O bond length.