  Edge-on spiral galaxies often have stellar disks with relatively sharp
truncations and warped HI-layers in the outer parts. Warps appear to start
preferentially near the optical boundaries of the disks. Here we make a
comparative study of warps and truncations in edge-on galaxies. The
Garc\'{\i}a-Ruiz et al. (2002) sample with extensive HI-mapping is complemented
with luminosity distributions from the Sloan Digital Sky Survey. The method to
identify truncations has been tested using the sample of edge-on galaxies of
van der Kruit & Searle. Results are: (i.) The majority (17 out of 23) of the
galaxies show evidence for truncations. (ii.) When an HI-warp is present it
starts at 1.1 truncation radii. (iii.) This supplements the rules for warps
formulated by Briggs (1990), if the Holmberg radius is replaced for edge-on
systems with the truncation radius. (iv.) The truncation radius and the onset
of the warps coincide radially with features in the rotation curve and the HI
surface density. The latter is also true for less inclined systems. (v.) Inner
disks are very flat and the onset of the warp just beyond the truncation radius
is abrupt and discontinuous. These findings suggest that the inner flat disk
and the outer warped disk are distinct components with quite different
formation histories, probably involving quite different epochs. The inner disk
forms initially and the warped outer disk forms as a result of much later
infall of gas with a higher angular momentum in a different orientation. In an
appendix the Holmberg radius is discussed. Contrary to what is often assumed
Holmberg radii are not corrected for inclination.
