  Observations of black hole binaries (BHBs) have established a rich
phenomenology of X-ray states. The soft states range from the low variability,
accretion disc dominated thermal state (TD) to the higher variability,
non-thermal steep power law state (SPL). The disc component in all states is
typically modeled with standard thin disc accretion theory. However, this
theory is inconsistent with optical/UV spectral, variability, and gravitational
microlensing observations of active galactic nuclei (AGNs), the supermassive
analogs of BHBs. An inhomogeneous disc (ID) model with large (~0.4 dex)
temperature fluctuations in each radial annulus can qualitatively explain all
of these AGN observations. The inhomogeneity may be a consequence of
instabilities in radiation dominated discs, and therefore may be present in
BHBs as well. We show that ID models can explain many features of the TD and
SPL states of BHBs. The observed relationships between spectral hardness, disc
fraction, and rms variability amplitude in BHBs are reproduced with temperature
fluctuations similar to those inferred in AGNs, suggesting a unified picture of
luminous accretion discs across orders of magnitude in black hole mass. This
picture can be tested with spectral fitting of ID models, X-ray polarization
observations, and radiation MHD simulations. If BHB accretion discs are indeed
inhomogeneous, only the most disc dominated states (disc fraction > 0.95) can
be used to robustly infer black hole spin using current continuum fitting
methods.
