The four sources that exhibit harmonic pairs of frequencies in a 3:2 ratio (see §6.2) suggest that HFQPOs arise from some type of resonance mechanism (Abramowicz & Kluzniak 2001; Remillard et al. 2002a). As a second example, one recent MHD simulation provides evidence for resonant oscillations in the inner disk (Kato 2004); in this case, however, the coupling relation involves the azimuthal and radial coordinate frequencies. For example, the "parametric resonance" concept (Abramowicz et al. 2003; Kluzniak et al. 2004; Torok et al. 2005) describes oscillations rooted in fluid flow where there is a coupling between the radial and polar coordinate frequencies. If radiating blobs do congregate at a resonance radius for some reason, then ray tracing calculations have shown that GR effects can cause measurable features in the X-ray power spectrum (Schnittman & Bertschinger 2004).