In bridged bicyclic ring systems, two rings share more than two atoms. In these cases, there may be fewer than 2n isomers because of the structure of the system. For example, there are only two isomers of camphor (a pair of enantiomers), although it has two chiral carbons. In both isomers, the methyl and hydrogen are cis. The trans pair of enantiomers is impossible in this case, since the bridge must be cis. The smallest bridged system so far prepared in which the bridge is trans is the [4.3.1] system; the trans ketone 80 has been prepared.