In this case there are four isomers, since both the trans and the cis (which has also been prepared) are pairs of enantiomers. When one of the bridges contains a substituent, the question arises as to how to name the isomers involved. When the two bridges that do not contain the substituent are of unequal length, the rule generally followed is that the prefix endo- is used when the substituent is closer to the longer of the two unsubstituted bridges; the prefix exo- is used when the substituent is closer to the shorter bridge; for example,