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 $4n$ electrons it is stabilized (aromatic), whereas with $4n+2$ it is destabilized (antiaromatic). In the excited state $4n+2$ Möbius π-electron systems are stabilized, and $4n$ systems are destabilized. No examples of ground-state Möbius π-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.