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4.2 Transitions Between Ordinary and Barred Spirals

Many galaxies combine features of pure ordinary spirals (such as NGC 628, Atlas, p. 29) and pure barred systems (e.g., NGC 1300, Atlas, p. 45). Examples are NGC 4579 (Atlas, p. 13), 5236 (Atlas, p. 28), and 3504 (Atlas, p. 46), which Hubble and Sandage denoted by the mixed symbols Sb/SBb, Sc/SBb, and SBb/Sb, respectively, where the leading symbol gives the dominant type. De Vaucouleurs uses a more convenient and symmetrical notation by (1) adding a type-symbol A to the ordinary spirals, (2) retaining the symbol B for the barred spirals, and (3) adding the mixed symbol AB for the transition cases. De Vaucouleurs's notation for the three examples quoted are SABb (4579), SABc (5236), and SABab (3504), where the dominant type is underlined.