What type of isomer are tautomers specifically classified as?

Tautomers are classified as constitutional isomers because they differ in the connectivity of their atoms. Constitutional isomers, also known as structural isomers, have the same molecular formula but vary in how the atoms are bonded together, which leads to different structural forms. Tautomers specifically involve a rapid equilibrium between two isomers that differ in the position of a proton and the double bond; for example, keto-enol tautomerism is a well-known illustration where the keto form has a carbonyl group (C=O) while the enol form has an alcohol (C=C-OH).

This classification is distinct from stereoisomers, which share the same connectivity of atoms but differ in the spatial arrangement of those atoms. Geometric isomers and enantiomers represent more specific subcategories of stereoisomers, dealing with cis-trans configurations and non-superimposable mirror images, respectively. Therefore, tautomers being a subset of constitutional isomers highlights their unique characteristic of differing primarily in the locations of double bonds and protons within the same molecular framework.