Zimmerman envisioned 4 π-electrons in allene like Heilbronner's Möbius-cyclobutadiene. When allene is in its D 2d ground-state structure the p-orbitals achieve a Möbius topology with one out-of-phase overlap. This corresponds to a Heilbronner 4N system as illustrated in Figure , which provides an intuitive interpretation for structural differences: When the sp -hybridized end-groups of a linear chain of sp-hybridized atoms are in coplanar orientation, its coarctate basis has Hückel topology and follows the 4N+2 rule; if the relative orientation is perpendicular the basis has Möbius topology and follows the 4N rule. By considering the number of π-electrons we can rationalize the end-group orientation of allenes, cumulenes and polyynes. Evident from the torsion energy profile in Figure , neutral allene is stable with perpendicular end-groups and becomes increasingly unstable as the end-groups or rotated towards coplanarity. Conversely, [3]cumulene has co-planar end-groups in order to achieve stable 4n+2 Hückel topology. Diaminoacetylene has two additional π-electrons and consequently the aminegroups are perpendicularly oriented to achieve Möbius topology. Various nitrogen-terminated polyynes have perpendicular endgroups, and exceptions relating to linear molecules seem rare. The mnemonic also accounts for the disputed cases of chemical bonding in diboracumulenes and borynes reported by Braunschweig and co-workers. A brief discussion of linear boron system and tolane are provided in ESI part 2.