  We propose a simple interpretation of the rotation period data for solar- and
late-type stars. The open cluster and Mt. Wilson star observations suggest that
rotating stars lie primarily on two sequences, initially called I and C. Some
stars lie in the intervening gap. These sequences, and the fractional numbers
of stars on each sequence evolve systematically with cluster age, enabling us
to construct crude rotational isochrones allowing `stellar gyrochronology', a
procedure, upon improvement, likely to yield ages for individual field stars.
The age and color dependences of the sequences allow the identification of the
underlying mechanism, which appears to be primarily magnetic. The majority of
solar- and late-type stars possess a dominant Sun-like, or Interface magnetic
field, which connects the convective envelope both to the radiative interior of
the star and to the exterior where winds can drain off angular momentum. These
stars spin down Skumanich-style. An age-decreasing fraction of young G, K, and
M stars, which are rapid rotators, possess only a Convective field which is not
only inefficient in depleting angular momentum, but also incapable of coupling
the surface convection zone to the inner radiative zone, so that only the outer
zone is spun down, and on an exponential timescale. These stars do not yet
possess large-scale dynamos. The large-scale magnetic field associated with the
dynamo, apparently created by the shear between the decoupled radiative and
convective zones, (re)couples the convective and radiative zones and drives a
star from the Convective to the Interface sequence through the gap on a
timescale that increases as stellar mass decreases. (Abstract is truncated
here.)
