Although the semi-localized textbook σ-π separation model describes the bonding of allene using a pair of localized orthogonal π orbitals, the full molecular orbital description of the bonding is more subtle. The symmetry-correct doubly-degenerate HOMOs of allene (adapted to the D2d point group) can either be represented by a pair of orthogonal MOs "or" as twisted helical linear combinations of these orthogonal MOs. The symmetry of the system and the implied degeneracy of these orbitals result in both descriptions being correct (in the same way that there are infinitely many ways to depict the doubly degenerate HOMOs and LUMOs of benzene that correspond to different choices of eigenfunctions in a two-dimensional eigenspace). However, this degeneracy is lifted in substituted allenes, and the helical picture becomes the only symmetry-correct description for the HOMO and HOMO–1 of the C2-symmetric 1,3-dimethylallene. This qualitative MO description extends to higher odd-carbon cumulenes (e.g., 1,2,3,4-pentatetraene).