For double bonded molecules, Cahn–Ingold–Prelog priority rules (CIP rules) are followed to determine the priority of substituents of the double bond. If both of the high priority groups are on the same side of the double bond (cis configuration), then the stereoisomer is assigned the configuration Z (zusammen, German word meaning "together").  If the high priority groups are on opposite sides of the double bond ( trans configuration ), then the stereoisomer is assigned the configuration E (entgegen, German word meaning "opposed")