The Hubble tuning fork is useful because it provides a visual representation of information Hubble (1926) had only stated in words. The fork contains an implication of continuity. For example, it does not rule out that there might be galaxies intermediate in characteristics between an "Sa" or "Sb" spiral, or between a normal "S" spiral and a barred "SB" spiral. Continuity along the elliptical galaxy sequence was always implied as a smooth variation from round ellipticals (E0) to the most flattened ellipticals (E7). Sandage (1961) describes the modifications that made the Hubble system more three-dimensional: the introduction of the (r) (inner ring) and (s) (pure spiral) subtypes. Continuity even with this characteristic was possible, using the combined subtype (rs). Thus, already by 1961, the Hubble classification system had become much more complicated than it was in 1926 or 1936. The addition of the S0 class was one reason for this, but the (r) and (s) subtypes were another.
In the Hubble-Sandage classification, it became common to denote galaxies on the left part of the Hubble sequence as "early-type" galaxies and those on the right part as "late-type" galaxies. By the same token, Sa and SBa spirals became "early-type spirals" while Sc and SBc spirals became "late-type spirals." Sb and SBb types became known as "intermediate-type spirals." The reason for these terminologies was convenience and borrows terminology often used for stars. Young, massive stars of spectral classes O and B were known as "early-type stars" while older stars of cooler spectral types were known as "late-type stars." Hubble stated that his use of these temporal descriptions for galaxies had no evolutionary implications. An irony in this is that it eventually became clear that early-type galaxies are dominated by late-type stars, while late-type galaxies often have significant numbers of early-type stars.
De Vaucouleurs (1959) took the idea of continuity of galaxy morphology a step further by developing what he referred to as the classification volume (Figure 3). In this revision of the Hubble-Sandage (1961) classification, galaxy morphology represents a continuous sequence of forms in a three-dimensional volume with a long axis and circular cross-sections of varying size. The long axis of the volume is the stage, or type, and it represents the long axis of the original Hubble tuning fork. The short axes are the family and the variety, which refer to apparent bar strength and the presence or absence of an inner ring, respectively. In addition to Hubble's original stages E, Sa, Sb, and Sc, the classification volume includes new stages: late ellipticals: E+, "very late" spirals: Sd, "Magellanic spirals": Sm, and "Magellanic irregulars": Im. The S0 class is included in the same position along the sequence, between E's and spirals, but is subdivided into three stages. The characteristics defining individual stages are described further in section 5.2.
Figure 3. de Vaucouleurs's (1959) classification volume, a revision and extension of the Hubble tuning fork. The three dimensions are the stage (Hubble type), the family (apparent bar strength), and the variety (presence or absence of an inner ring).
The stage is considered the most fundamental dimension of the classification volume because measured physical parameters, such as integrated color indices, mean surface brightnesses, and neutral hydrogen content correlate well with position along the sequence (e. g., Buta et al. 1994). Early-type galaxies tend to have redder colors, higher average surface brightnesses, and lower neutral hydrogen content than late-type galaxies. The family and variety axes of the classification volume indicate the considerable variations in morphology at a given stage. A famous sketch of families and varieties near stage Sb, drawn by de Vaucouleurs himself during a cloudy night at McDonald Observatory circa 1962, is shown in Sandage (1975) and in Figure 1.13 of the dVA. The classification volume is broader in the middle compared to the ends because the diversity of galaxy morphology is largest at stages like S0/a and Sa. Bars and rings are often most distinct and most recognizable at these stages. Such features are not characteristic of E galaxies, so the volume must be narrow at that end. Along the S0 sequence, bars and rings are barely developed among the earliest S0s (S0-) and well-developed among the late S0s (S0+), thus the volume begins to broaden. Among very late-type galaxies, Sd, Sdm, Sm, and Im, bars are actually very frequent, but closed rings (r) are not. Thus, the volume narrows at that end as well.
For the purposes of illustrating morphology, blue light digital images converted to units of magnitudes per square arcsecond are used when available. This approach is described in the dVA, and requires calibration of the images, usually based on published photoelectric multi-aperture photometry. In addition, some of the illustrations used (especially in section 15) are from the Sloan Digital Sky Survey or from other sources. These are not in the same units but still provide excellent illustrations of morphology. SDSS color images differ from dVA images mainly in the central regions, where SDSS images sometimes lose detail.
Unlike the dVA, the scope of this article extends beyond the traditional UBVRI wavebands. It is only during the past 20 years that significant morphological information has been obtained for galaxies outside these bands, mostly at mid- and far-ultraviolet and mid-IR wavelengths from space-based observatories capable of imaging in these wavelengths to unprecedented depths, providing a new view of galaxy morphology that is only beginning to be explored. A useful review of many issues in morphology is provided by van den Bergh (1998).