The structure of barrelene. Barrelene is thought of having two rings and mirrors what is found in the MCB and reflected in its systematic name (bicyclo[2.2.2]octa-2,5,7-triene). The choice of which two rings is arbitrary. Barrelene has three relevant cycles and no essential cycles. In this simple example the choice of which two cycles is irrelevant as they are symmetric. This would no longer be the case if the structure was hetereocyclic or had exocyclic group added.