  Stellar rotation is a strong function of both mass and evolutionary state.
Missions such as Kepler and CoRoT provide tens of thousands of rotation
periods, drawn from stellar populations that contain objects at a range of
masses, ages, and evolutionary states. Given a set of reasonable starting
conditions and a prescription for angular momentum loss, we address the
expected range of rotation periods for cool field stellar populations. We find
that cool stars fall into three distinct regimes in rotation. Rapid rotators
with surface periods less than 10 days are either young low-mass main sequence
(MS) stars, or higher mass subgiants which leave the MS with high rotation
rates. Intermediate rotators (10-40 days) can be either cool MS dwarfs,
suitable for gyrochronology, or crossing subgiants at a range of masses.
Gyrochronology relations must therefore be applied cautiously, since there is
an abundant population of subgiant contaminants. The slowest rotators, at
periods greater than 40 days, are lower mass subgiants undergoing envelope
expansion. We identify additional diagnostic uses of rotation periods. There
exists a period-age relation for subgiants distinct from the MS period-age
relations. There is also a period-radius relation that can be used as a
constraint on the stellar radius, particularly in the interesting case of
planet host stars. The high-mass/low-mass break in the rotation distribution on
the MS persists onto the subgiant branch, and has potential as a diagnostic of
stellar mass. Finally, this set of theoretical predictions can be compared to
extensive datasets to motivate improved modeling.
