There are two requirements for cis-trans isomerism:
Rotation must be restricted in the molecule, the double bond provides this.
There must be two nonidentical groups on each doubly bonded carbon atom.
But they don't have to be hydrogen you could have cis/trans isomers of Ch3ClC=CClCh3
With for example the methyl groups on either the same side (cis) or opposite sides (trans).
Hope that helps!