In chemistry, a methine group or methine bridge is a trivalent functional group =CH−, derived formally from methane. It consists of a carbon atom bound by two single bonds and one double bond, where one of the single bonds is to a hydrogen. The group is also called methyne or methene; its IUPAC systematic name is methylylidene or methanylylidene
This group is sometimes called "methylidyne", however that name belongs properly to either the methylidyne group ≡CH (connected to the rest of the molecule by a triple bond) or to the methylidyne radical ⫶CH (the two atoms as a free molecule with dangling bonds).
Two or more methine bridges can overlap, forming a chain or ring of carbon atoms connected by alternating single and double bonds, as in piperylene H
3, or the compound
Every carbon in this molecule is a methine carbon, except for three; two that are attached to the two nitrogens and not to any hydrogens, and the one attached to the nitrogen, which is attached to two hydrogens (far right). There is a five carbon poly-methine chain in the center of this molecule.