The classification systems in use today are all in some way related to the system described in Hubble's (1926) paper. A detailed review of several of these, in particular Hubble's revised system, de Vaucouleurs' revised Hubble system, van den Bergh's modified Hubble (or DDO) system with luminosity classes, Morgan's spectral form classification (or Yerkes) system, and Vorontsov-Velyaminov's purely descriptive (or MCG) system, has already been provided by Sandage (1975).
The only system which has changed significantly since 1975 is van den Bergh's modified Hubble system. For various reasons, van den Bergh (1976) disagreed with Hubble's placement of S0's in the "transition region" between ellipticals and spirals, and instead proposed placing S0's in a sequence parallel to spirals (called the RDDO system). His sequence of modified Hubble types uses bulge-to-disk ratio as the main classification criterion, while Hubble's final division of spiral types was based principally on the appearance of the arms. Using B/D ratio as the sole stage criterion, van den Bergh identified transition cases between S0's and normal spirals which appeared to be spirals with little star formation in the arms. These were given the term "anemics" and were assumed to be poor in HI (see Bothun and Sullivan, 1980). Van den Bergh, Pierce, and Tully (1990) have recently discussed the application of a further modified RDDO system to CCD images of galaxies.
Revisions to other systems are less drastic. Slight revisions to the Yerkes system are described by Morgan, Kayser, and White (1975), while Sandage and Brucato (1979) discuss refinements to the classification system in Sandage (1961). The Revised Shapley-Ames catalogue (RSA, Sandage and Tammann, 1987), which is one of the main applications of Hubble's revised system, now recognizes Sd and Sm types as in the de Vaucouleurs revised Hubble system. In addition, what de Vaucouleurs has called I0 and which was assigned a coded numerical stage T = 0 in the Second Reference Catalogue of Bright Galaxies (RC2, de Vaucouleurs et al., 1976), is now regarded as a special class of objects outside the scope of the Hubble sequence (Sandage and Brucato, 1979; see section 7.3).