  The northwest side of the disk of M31 is known to be the near side because of
the differential reddening of globular clusters found from their photographic
photometry. This paper reports a simple geometric model to evaluate the
visibility of the effect and its application to published CCD photometry on
globular cluster systems of three spiral galaxies, M31, M33, and NGC253. The
color difference of globular cluster systems due to differential reddening was
confirmed for M31 and NGC253; however, the data for M33 were insufficient. The
analysis reaffirms the currently adopted interpretation that the side on the
minor axis of the galactic disk, where more conspicuous dust features and
interstellar reddening are visible, is the nearer side to us and provides an
additional basis for using spiral galaxies with identified spiral windings,
S-wise or Z-wise, to study the large-scale spin distribution of galaxies in the
universe.
