Instead of being compositionally disordered, two types of atoms can instead form ordered arrangements. For example, the zincblende structure is an ordered version of the diamond structure. It can further be expanded to the chalchopyrite structure when the cations are ordered. Chalchopyrite (CuFeSe 2 ) can be viewed as a doubled zincblende lattice, where the Cu and Fe cations order. Now the ordering of the ions causes the unit cell to enlarge, lowering the symmetry, and changing the space group, with concurrent changes to the diffraction pattern. Another well-known example of such ordered superstructures is the double perovskite structure. In the case of alloys, Heusler alloys are a common example of ordered intermetallic compounds. Whether a compound has ordered or disordered atoms can often, but not always, be distinguished by XRD. The larger unit cells and/or lower symmetry of ordered compounds may result in additional diffraction peaks, or changes in intensity of peaks. If the ordered ions have very similar x-ray scattering factors, which are determined by the number of electrons in the ion, then XRD may not be able to detect their ordering, and may not be able to distinguish between a material with compositionally disordered ions and one where the same ions are ordered. Interestingly, it seems that many of the predicted new materials, both in Materials Project and the larger Google's GNoMe, fall in the category of structurally new materials. We have certainly not looked through all predicted new materials, not even a large fraction. By focusing on those that were picked as synthetic targets int the A-lab paper, we see a clear trend. The predicted new materials can very often be derived from known compounds, in which ions were ordered, rather than fractionally occupied and disordered, within the same aristotype as the known parent. If these ordered structures were synthesized, many indeed would qualify as a new crystallographic compounds. If the key characteristic that distinguishes a new material from a known one is cation order, then the cation order needs to be proven to be real, as otherwise the material would be identical to, or a very similar doped version of, the already known disordered version.