What is the cis-trans notation?

cis means on this side; trans means across or on opposite sides.

You can use the cis/trans notation when atoms are restricted from rotating around a bond. There is restricted rotation in alkenes and in rings.

Double Bonds

The alkene must have two different groups on each carbon atom of the double bond, and there must be one of the same group on each carbon atom.

The structure with the pink groups on opposite sides is trans. The structure with the pink groups on the same side is cis.

One of the structures below has the two chlorine atoms locked on opposite sides of the double bond. This is the trans isomer.

The other structure has the two chlorine atoms locked on the same side of the double bond. This is the cis isomer.

The compound below is trans, because it has CH₂CH₃ groups on opposite sides of each end of the double bond

Rings

Cyclic compounds can also display cis/trans isomerism. Consider, for example, 1-bromo-2-methylcyclopentane.

Structure A is cis, because both groups are on the same side of the ring. Structure B is trans, because the groups are on opposite sides of the ring.

For more complicated structures, we must use the E/Z notation, but that's a topic for another day.