isomerism of the general form: where the isomers (called tautomers) are readily interconvertible; the atoms connecting the groups X, Y, Z are typically any of C, H, O or S, and 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 Y and Z may be replaced by a ring, when the phenomenon is called ring-chain tautomerism, as in: