The cis / trans nomenclature is effective only when the alkene has two different groups on each carbon atom of the double bond, and each carbon has one of the same group. The structure with pink groups on opposite sides is trans.
In the nomenclature, “cis” is used to distinguish the isomer in which two identical groups (e.g., the two chlorides in 1,2-dichlorocyclopentane) are oriented in the same direction to the ring plane and trans to distinguish the isomer as indicated in opposite directions. [You may also hear organic chemists saying that “chlorides are cis to each other” or “hydrogen is cut to one another.”]
A common name for these so-called “cis-trans” isomers is “geometric isomers”. [Those IUPAC arguments actually discourage the term “geometric isomers” and, for once, they agree: the term is somewhat redundant and may cause confusion. In the rest of this post I will only use the term “cis-trans” isomers. ]
For cis-trans-isomerism to exist in rings, we need two conditions: two (and only two) carbon atoms each bearing each unidentified substituent on top and below the ring the two carbon atoms have at least one of these substituents in common
In 1,2-dichlorocyclopentane it has been seen that each C-1 and C-2 have non-identical substituents (H and Cl) above and below the ring and each of them has at least one substituent in common (in fact, they have two substituents in common: H and Cl).
Here is another example: cis- and trans-1-ethyl-2-methylcyclobutane. Note that each of them has two carbon atoms, each having non-identical substituents above and below the ring (H and CH3, H and CH2CH3). They also have at least one common substitute (H). Thus, cis-1-ethyl-2-methylcyclohexane may be referred to as an isomer in which the two hydrogens are directed in the same direction and the trans where they are directed in opposite directions.
