Tautomers are structural isomers that interconvert from one form to another. The most common, and best-known, type of tautomerism occurs by proton migration. Other forms that involve equilibrium between ring and chain form with and without proton migration are called ring-chain and valence tautomers. In textbooks and literature, the term "readily" is often used to describe the process. However, this is not correct in all cases. In the case of solid forms of drug molecules, the transformation from one form to another is very slow due to the difficult migration of protons.