A rule proposed by the German-Swiss chemist Edgar Heilbronner (1921–2006) in 1964 which states that in cyclic annulenes with Möbius strip conformations the electron counting rule for stability is the opposite to the Hückel rule; in other words, aromaticity for a Möbius strip cyclic annulene occurs when there are 4n pi electrons and anti-aromaticity occurs when there are (4n + 2) pi electrons. There is experimental evidence for the correctness of Heilbronner’s rule.