The E-Z Convention for Cis- Trans Isomers

Of course, we could have started with the top-priority group at either of the other two bonds to give the following structures, which are equivalent to the previous structures  

Before we continue with other aspects of chirality, let us digress briefly to describe a useful extension of the Cahn-lngold-Prelog system of nomenclature to cis-trans isomers. Although we can easily use cis-trans nomenclature for 1,2-dichloroethene or 2-butene (see Sec. 3.5), that system is sometimes ambiguous, as in the following examples  

Piane-poiarized iight is a light beam consisting of waves that vibrate in parallel planes. 

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