An element's fine structure is its production of distinct spectral lines due to very small differences in energy states of atoms due to differences in electron spin and relativistic effects. When a spectral energy distribution is viewed at a low spectral resolution and/or there is significant line broadening, these fine structure lines may appear as a single line and the term gross structure is used to indicate the spectral lines apparent if the fine structure is ignored or thus hidden, i.e., lines defined by differences in the energy levels of electron shells. When the fine structure can be discerned, gross structure lines can be seen as multiple fine lines. The fine lines' energy level difference (thus photon energy and frequency) is much smaller than that between gross structure lines, the ratio of the difference being on the order of the square of the product of the atomic number and the fine structure constant, the latter which is approximately 1/137.
For carbon, the fine structure lines are forbidden lines and are listed as [CI] (490 GHz and 810 GHz) and [CII] (1.9 THz) for neutral and ionized carbon. Oxygen fine structure lines include 63 μm.
The hyperfine structure represents a mechanism for even smaller differentiations between lines: for hydrogen, the hyperfine structure of energy level n=1 results in the 21-cm line and the fine structure of energy level n=2 is results in a 28.37 cm line.