Most spiral galaxies are, to a good approximation, oblate spheroids that can be arranged on the sequence Sa–Sb–Sc. However, a significant minority exhibit bar-like structures and may be placed on the SBa–SBb–SBc tine of Hubble's ‘tuning fork’ diagram. Objects, with less pronounced bars, can be arranged on the intermediate sequence S(B)a–S(B)b–S(B)c between normal and barred spirals. Some disks appear to be globally oval (Kormendy 1982, p. 135). Such oval disks are of interest because, like bars, they represent non-axisymmetric distortions of the gravitational potential. An excellent review on the dynamics of barred galaxies has been given by Sellwood & Wilkinson (1993). Infrared imaging shows that some galaxies contain small IR bars (Frogel, Quillen & Pogge 1996). When such small inner bars are found in galaxies with large outer bars there is no correlation between the position angles of the large and small bars.

Bars are important to the dynamical evolution of galaxies (Kormendy 1982, 1993) because they can (a) lose angular momentum to cold disks and dark halos, and (b) gain angular momentum from rapidly rotating bulges. Furthermore, transfer of significant amounts of gas by bars to the nuclear regions of galaxies will increase the central galactic mass concentration, which in turn will make the galactic disk less prone to the development of bar-like distortions. In other words stellar bars can self-destruct (or transform themselves into lenses) by transporting too much gas to the nuclear regions of their parent galaxies.