  We present the results of a detailed study of vertical surface brightness
profiles of edge-on disk galaxies. Although the exponential disk scale height
is constant to first order approximation, we show that for the large majority
of galaxies in our sample, the scale height increases with distance along the
major axis. The effect is strongest for early-type galaxies, where the increase
of the scale height can be as much as a factor of 1.5 per scalelength, but is
almost 0 for the latest-type galaxies. The effect can be understood if
early-type disk galaxies have thick disks with both scale lengths and scale
heights larger than those of the dominant disk component. Its origin appears to
be linked to the processes that have formed the thick disk.
