Hubble  introduced S0 galaxies as a morphological transition case between ellipticals and early-type spirals. A detailed description of these lenticular galaxies and a possible sub-classification scheme is given in ; see the table below. Others [9, 196] have suggested that S0's actually run parallel to the ordinary spiral stages but form one extreme of gas-depleted disk systems (S0a, S0b, S0c) coupled to the gas-rich spirals by yet another intermediate sequence of ``anaemic'' spirals (Aa, Ab, Ac). de Vaucouleurs  suggests that lenticulars may be subdivided as well, and his ``early-type'' and ``late-type'' classification is also outlined below.
|(a) de Vaucouleurs |
|S0-||Traces of structure can be found in the smooth lens and envelope; a small nucleus may be present.||NGC
|S0°||A weak trace of a ring appears at the edge of the lens; a distinct nucleus and envelope may be present.||NGC 1553|
|S0+||A well defined ring is present separating the inner nuclear bulge from outer incipient spiral structure.||NGC 2855|
|(b) Sandage |
|S01||The existence of an outer envelope, flattened to a fundamental plane, defines this sub-type; the ellipticals of the central section of such galaxies are flatter than E7.||NGC 1201|
|S02||The first appearance of a circular absorption pattern or a true depletion of material in the envelope defines this intermediate sub-type.||NGC 0542|
|S03||A strong internal circular ring defines this extreme of the S0 class.||NGC 5866|
Note: There are in fact many non-ringed S0° and S0+ galaxies also, see Table 3 (pg.35) in .