Double-Barred Galaxies
  I present a brief review of what is known about double-barred galaxies, where
a small ("inner") bar is nested inside a larger ("outer") bar; the review is
focused primarily on their demographics and photometric properties. Roughly 20%
of S0--Sb galaxies are double-barred; they may be rarer in later Hubble types.
Inner bars are typically ~ 500 pc in radius (~ 12% the size of outer bars), but
sizes range from ~ 100 pc to > 1 kpc. The structure of at least some inner bars
appears very similar to that of outer bars (and single large-scale bars).
Direct and indirect evidence all support the hypothesis that inner bars rotate
independently of outer bars, although actual pattern speeds for inner bars are
poorly constrained. Finally, I note that inner bars do not appear to promote
nuclear activity.
