The luminosity class introduced by
Van den Bergh (1960)
has been numerically coded between 1 and 9 with the extension introduced by
H.G. Corwin (SGC, 1985) between 9 and 11.
The luminosity class is coded on a continuous scale which is related to
the original classification as:
The original nomenclature was chosen to match the Yerkes system of stellar luminosities.