ALT The graphic shows three regioisomers of diazines in the top row and shows a sketch of a pill with an attached arene. The latter alludes to the fact that the ability to prepare individual regioisomers permits researchers to optimize the position of nitrogen atoms in arene rings for applications in medicinal chemistry. The bottom row shows simplistic representations of four different structomers of metal-organic frameworks. All four structomers have the same composition and are isostructural, but differ in how "active" and "inactive" metal sites are arranged within the structure. Selective preparation of individual structomers thus permits one to optimize metal arrangements in extended structures.