A monocyclic array of orbitals in which there is a single out-of-phase overlap (or, more generally, an odd number of out-of-phase overlaps) reveals the opposite pattern of aromatic character to Hückel systems; with \(4\, n\) electrons it is stabilized (aromatic), whereas with \(4\, n+2\) it is destabilized (antiaromatic). In the excited state \(4\, n+2\) Möbius $\ce{\uppi\!\mbox{-}electron}$ systems are stabilized, and \(4\, n\) systems are destabilized. No examples of ground-state Möbius $\ce{\uppi\!\mbox{-}electron}$ systems are known, but the concept has been applied to transition states of pericyclic reactions. The name is derived from the topological analogy of such an arrangement of orbitals to a Möbius strip.