Compounds that contain two fixed double bonds (usually assumed to be between carbon atoms). Dienes in which the two double-bond units are linked by one single bond are termed conjugated, e.g. CH₂=CH–CH=CH₂ buta-1,3-diene. Dienes in which the double bonds are adjacent are called cumulative, e.g. CH₃–CH=C=CH₂ buta-1,2-diene. Those in which one or more of the unsaturated carbon atoms is replaced by a heteroatom may be called heterodienes.