  Projected rotational velocities have been measured for 216 B0--B9 stars in
the rich, dense h and chi Persei double cluster, and compared with the
distribution of rotational velocities for a sample of field stars having
comparable ages and masses. For stars that are relatively little evolved from
their initial locations on the Zero Age Main Sequence (3-5 solar masses) the
mean vsini measured for the h and chi Per sample is slightly more than 2 times
larger than the mean determined for field stars of comparable mass, and the
cluster and field vsini distributions differ with a high degree of
significance. For somewhat more evolved stars with masses in the range 5-9
solar masses, the mean vsini in h and chi Per is 1.5 times that of the field;
the vsini distributions differ as well, but with a lower degree of statistical
significance. For stars that have evolved significantly from the ZAMS (those
with masses in the range 9-15 solar masses), the cluster and field star means
and distributions are only slightly different. We argue that both the higher
rotation rates and the pattern of rotation speeds as a function of mass that
differentiate main sequence B stars in h and chi Per from their field analogs
were likely imprinted during the star formation process. We speculate that
these differences may reflect the effects of the higher accretion rates that
theory suggests are characteristic of regions that give birth to dense
clusters, namely: (a) higher initial rotation speeds; (b) higher initial radii
along the stellar birthline, resulting in greater spinup between the birthline
and the ZAMS; and (c) a more pronounced maximum in the birthline radius-mass
relationship that results in differentially greater spinup for stars that
become mid- to late- B stars on the ZAMS.
