  We show that viscous, transonic accretion disks which start with
sub-Keplerian angular momentum at the outer boundary reproduce the features of
the so-called thick, thin, slim and other accretion disk models depending upon
the accretion rates and viscosity parameters. These disks are advection
dominated in the pre-shock region and rotation dominated in the post-shock
region when viscosity is less than a critical value. For a higher viscosity the
shock disappears, and the flow has significant radial velocity only close to
the black hole.
