cis-1,2-dichloroethene (left); trans-1,2-dichloroethene (right).
Cis-trans isomerism is a form of isomerism associated with compounds containing a double bond. Like groups in such compounds may be either on the same side of the plane of the double bond (cis-form) or on opposite sides (trans-form). E.g., maleic acid and fumaric acid are, respectively, cis- and trans-forms. See also stereoisomerism.