An unusual kind of stereoisomerism termed E/Z isomerism without a double bond or generalised E/Z isomerism is discussed for the first time. Examples are given of pairs of synthesized and separated compounds related by this kind of isomerism (4a/5a and 12a/15a), which were obtained by an unusual rearrangement of substituents on a bicyclobutane ring system. Conditions for the occurrence of generalised E/Z isomerism are defined.