text: Prototropy is the most common form of tautomerism and refers to the relocation of a hydrogen atom. Prototropic tautomerism may be considered a subset of acid-base behavior. Prototropic tautomers are sets of isomeric protonation states with the same empirical formula and total charge. Tautomerizations are catalyzed by:
  * bases, involving a series of steps: deprotonation, formation of a delocalized anion (e.g., an enolate), and protonation at a different position of the anion; and
  * acids, involving a series of steps: protonation, formation of a delocalized cation, and deprotonation at a different position adjacent to the cation).
  Two specific further subcategories of tautomerizations:
  *Annular tautomerism is a type of prototropic tautomerism wherein a proton can occupy two or more positions of the heterocyclic systems found in many drugs, for example, 1H- and 3H-imidazole; 1H-, 2H- and 4H- 1,2,4-triazole; 1H- and 2H- isoindole.
  *Ring–chain tautomers occur when the movement of the proton is accompanied by a change from an open structure to a ring, such as the open chain and cyclic hemiacetal (typically pyranose or furanose forms) of many sugars.  (See .)  The tautomeric shift can be described as H−O&thinsp;⋅&thinsp;C=O ⇌ O−C−O−H, where the "⋅" indicates the initial absence of a bond.