How can E-Z isomerism occur if there is restricted rotation about the double bond? Watch
However, if the groups are free to rotate around the C=C bond, then the idea of an E/Z isomerism would not even exist.
Therefore E/Z isomerism only occurs when there is restricted rotation about the C=C, because without that rotation, the groups are free to rotate around the Carbon bond, meaning that there would not be an isomerism in the first place.
A is at the same "end" as B, and C is at the same "end" as D
So there are two possibiities:
1. A is on the same long side as C,
2. A is on the same long side as D.