Isomerism of the general form: where the isomers (called tautomers) are readily interconvertible; the atoms connecting the groups $\ce{X}$, $\ce{Y}$, $\ce{Z}$ are typically any of $\ce{C}$, $\ce{H}$, $\ce{O}$ or $\ce{S}$, and $\ce{G}$ is a group which becomes an electrofuge or nucleofuge during isomerization. The commonest case, when the electrofuge is H+, is also known as 'prototropy'. Examples, written so as to illustrate the general pattern given above, include: keto-enol tautomerism, such as: The grouping Y may itself be a three-atom (or five-atom) chain extending the conjugation, as in: The double bond between $\ce{Y}$ and $\ce{Z}$ may be replaced by a ring, when the phenomenon is called ring-chain tautomerism, as in: