Left: He I energy levels in the parahelium (S = 0) state. The levels 6 ≤ n ≤ 9 are grouped into one singlet and one triplet level each and these are denoted as n_sing. For clarity, the energy scale changes as the energy levels become closer at E = 20 eV and E = 24 eV, emphasised here by the change in line style of the energy axis at these points. The dotted lines show electric dipole permitted transitions. Although not included (to prevent overcrowding of the diagram), the 6 ≤ n ≤ 9 singlet levels possess allowed transitions with all of the other levels. Right: He I energy levels in the orthohelium (S = 1) state. Note that the lowest excited energy states of both regimes are metastable.