The configuration about double bonds is undoubtedly best specified by the cis-trans notation when there is no ambiguity involved. Unfortunately, many compounds cannot be described adequately by the cis-trans system. Consider, for example, configurational isomers of 1-fluoro-1-chloro-2-bromo-2-iodo-ethene, \(9\) and \(10\). There is no obvious way in which the cis-trans system can be used: A system that is easy to use and which is based on the sequence rules already described for the \(R\),\(S\) system works as follows: An order of precedence is established for the two atoms or groups attached to each end of the double bond according to the sequence rules of Section 19-6 . When these rules are applied to 1-fluoro-1-chloro-2-bromo-2-iodoethene, the priority sequence is: at carbon atom 1, \(\ce{Cl} > \ce{F}\) at carbon atom 2, \(\ce{I} > \ce{Br}\) Examination of the two configurations shows that the two priority groups - one on each end - are either on the same side of the double bond or on opposite sides: The \(Z\) isomer is designated as the isomer in which the top priority groups are on the same side (\(Z\) is taken from the German word zusammen - together). The \(E\) isomer has these groups on opposite sides (\(E\), German for entgegen - across).\(^2\) Two further examples show how the nomenclature is used: This system is especially useful for oximes, which have the structural feature \(\ce{-C=N-OH}\). The two possible configurations at the double bond in the oxime of ethanal are \(11\) and \(12\): The cis-trans notation does not work well here, and structure \(11\) has the \(Z\) configuration and \(12\) the \(E\) configuration. In the older chemical literature, these stereoisomers were designated as syn and anti forms, but these names are really no better than cis and trans . \(^2\)It would have been simpler to remember if \(E\) stood for same side and \(Z\) for opposite side , but it is too late now.