  The BTZ black hole is geometrically finite. This means that its three
dimensional hyperbolic structure as encoded in its metric is in 1-1
correspondence with the Teichmuller space of its boundary, which is a two
torus. The equivalence of different Teichmuller parameters related by the
action of the modular group therefore requires the invariance of the
monodromies of the solutions of the wave equation around the inner and outer
horizons in the BTZ background. We show that this invariance condition leads to
the non-quasinormal mode frequencies discussed by Birmingham and Carlip.
