Requirements: Restricted rotation (C=C, C=N, or ring) AND different groups on each carbon of the restricted bond.

CIP Priority Rules:

1. Higher atomic number at first point of difference gets higher priority
2. If tied, move outward to next atoms (BFS-style)
3. Double bond = two single bonds to the same atom (phantom atoms)
4. Z (zusammen) = higher priority groups on SAME side; E (entgegen) = OPPOSITE sides

Key examples:

• 2-butene: cis (Z) and trans (E) forms. Trans is more stable (less steric strain).
• Oximes (C=N-OH): syn and anti forms exist.
• Cyclic compounds: 1,2-dimethylcyclohexane has cis/trans isomers.

Number of geometrical isomers: If a compound has n C=C bonds each capable of E/Z isomerism, max = 2^n.