4.3 The r and s Varieties
Barred spirals are of two dominant varieties: the r type in which the arms begin tangent to the external ring upon which the bar terminates (the purest example may be NGC 2523, Atlas, p. 48, and plate 6 of this chapter), and the s type in which the arms begin from the end of the bar (e.g., NGC 1300, Atlas, p. 45, and plate 6 here).
Recognition of the r and s varieties was made in the Hubble Atlas for all barred spirals by the notation SBb(r) for 2523, or SBb(s) for 1300. Mixed types such as 1073 (Atlas, p. 49, plate 6 here) were called sr or rs according to which variety dominates.
The same phenomenon is present in ordinary spirals, although it is somewhat more difficult to detect. Examples are 4274 Sa(r) (Atlas, p. 12), and 309 Sc(r) (Atlas, p. 32), and 5457 (M101) Sc(s) (Atlas, pp. 27 and 31), and especially 4321 (M100) Sc(s) (Atlas, pp. 28 and 31). The r-type spirals had previously been noted by Shapley and Paraskevopoulos (1940), by Randers (1940), by Vorontsov-Velyaminov (1965), and undoubtedly by many others.
De Vaucouleurs adopted the r and s distinction for all non-E galaxies, underlining the r or s symbol in the mixed variety according to the dominant form. For example, the transition galaxy NGC 5236 is classified as SAB (s)c in the revised system, while NGC 4579 (Atlas, p. 13; plate 5 here) is SAB(rs)b [classed Sb/SBb(rs) in the Hubble Atlas].