Tautomers are constitutional isomers that differ from each other in the locations of a hydrogen atom and one or more double bonds and interconvert rapidly.

If the hypothetical compounds A and B are tautomers and if, say, A forms as the product in a reaction, some of it is spontaneously and rapidly converted to B, some of which is converted back to A, instantaneously establishing equilibrium between A and B.  Thus, there is no independent existence of either A or B.

eg. 1:

eg. 2:

eg. 3:

see also tautomerism, tautomerization