It is well established now that bars are very often found in disc galaxies. One usually finds a bar fraction of ~ 2/3 , considering both prominent and weak bars, unless one is looking for bars in images observed at too short wavelengths , as the stellar content of bars is usually dominated by old, red stars. In fact, most bars fade away in ultra-violet images , although some bars can still be recognised (cf. NGC 1097). Even though the remaining ~ 1/3 of disc galaxies do not seem to harbour identifiable bars, they might still have less prominent non-axisymmetric distortions.
In addition, bars are seen in galaxies with a wide range of bulge-to-total ratio and mass, i.e. from lenticulars to irregulars. Thus, secular evolution processes induced by bars occur not only in disc-dominated galaxies with inconspicuous bulges. They also happen in bulge-dominated galaxies, which suggests the coexistence of classical, merger-built bulges, with bulges built from disc dynamical instabilities . But secular evolution can also happen without bars. Oval distortions in discs can also induce a substantial exchange of angular momentum from the inner to the outer parts of galaxies .
Finally, a recent development suggests that the total amount of mass within stars that reside in bars at z 0 is similar to that kept in stars belonging to classical bulges. A similar amount is confined to elliptical galaxies. Approximately 15% of the total mass in stars at z 0 is located in bars. Classical bulges and elliptical galaxies contain each a comparable fraction. This means that, as far as the stellar mass budget in the local universe is concerned, bars are as relevant as classical bulges and elliptical galaxies. The other ~ 1/2 of the stellar mass content at z 0 is in galaxy discs [36, 24].