  We compile optical and UV spectra of a sample of 'typical' short period black
hole X-ray transients in outburst. We also survey determinations of
interstellar extinction and distance in order to deredden spectra and compare
absolute fluxes. Hence we perform a comparative study of the broad-band
spectral energy distributions (SED). We find that given such a homogeneous
sample of typical sources, the optical SEDs form a relatively uniform set, all
exhibiting quasi-power-law spectra with Fnu proportional to nu^alpha, where
0.5<alpha<1.5 (steeper than the canonical nu^1/3 disk spectrum). All become
flatter in the UV, although there is more diversity here. The SEDs studied can
be broadly divided into two optical-UV spectral states. The UV-hard spectra,
e.g. A0620-00 and X-ray Nova Muscae 1991, continue to rise in the far-UV. The
UV-soft spectra, e.g. GRO J0422+32, drop off. XTE J1859+226 evolved from
UV-soft to UV-hard as it decayed indicating that this effect is a real
difference, not a dereddening artifact. All of the spectra can be fitted by a
generalized black body disk model with two forms of heating, resulting in the
two states. The UV-soft state is consistent with a disk illuminated by a
central point source, with irradiative heating dominating over viscous. The
UV-hard state is well described by a viscously heated disk, although this
requires very high mass flow rates in the case of Nova Muscae. Alternatively, a
UV-hard spectrum can be produced if the disk is illuminated by a vertically
extended X-ray source such as a central scattering corona or jet. [Abridged]
