In general, cis–trans stereoisomers contain double bonds or ring structures. In both cases the rotation of bonds is restricted or prevented. When the substituent groups are oriented in the same direction, the diastereomer is referred to as cis, whereas when the substituents are oriented in opposing directions, the diastereomer is referred to as  trans. An example of a small hydrocarbon displaying cis–trans isomerism is but-2-ene. 1,2-Dichlorocyclohexane is another example.