  The disk positions for galaxies of various morphological and nuclear-activity
types (normal galaxies, QSO, Sy, E/S0, low-surface brightness galaxies, etc.)
on the "central surface brightness - exponential scalelength" plane are
considered. The stellar disks are shown to form a single sequence on this plane
over a wide range of surface brightnesses (\mu_0(I)=12-25) and sizes (h=10 pc -
100 kpc). The existence of this observed sequence can probably be explaned by a
combination of three factors: a disk-stability requirement, a limited total
disk luminosity, and observational selection. The model by Mo et al. (1998) for
disk formation in the CDM hierarchical-clustering scenario is shown to
satisfactorily reproduce the salient features of the galaxy disk distribution
on the "\mu_0 - h" plane.
