While we focus on even [n]cumulenes in this paper, the helical MOs are also seen in appropriately substituted polyynes, odd [n]cumulenes, metallacumulenes orpolyynes, spiro-compounds, and cyclo-allenes. We choose to focus on the even [n]cumulenes as these have helical MOs in their ground state, and long molecules result in long helical MOs, which aids visualization.