Published online by Cambridge University Press: 12 April 2016
Numerous galaxies clearly show two misaligned nested stellar bars or one bar plus one structure with twisted isophotes. Some galaxies seem even to have three distinct triaxial structures. Recent optical and near-infrared multi-band observations show that the angle between the primary and secondary bars does not take any peculiar value. The best way to reconcile such observations with theory certainly consists in postulating that the secondary bar rotates faster than the primary one. Numerical simulations have demonstrated the viability of such systems, and both direct and counterrotating models with bars within bars can be computed. Bar-within-bar systems could play a significant role in fueling AGN’s and/or circumnuclear starbursts, since AGN’s and/or blue or Hα nuclear rings are frequently hosted by such galaxies.