ฺฺBecause chlorine has a higher atomic number than carbon, a -Cl substituent receives higher priority than a -CH3 group. Methyl receives higher priority than hydrogen, however, and isomer(a) is assigned E geometry because its high-priority groups are on opposite sides of the double bond. Isomer (b) has Z geometry because its high-priority groups are on "ze zame zide" of the double bond.