  We reexamine evolutionary channels for the formation of binary millisecond
pulsars in order to understand their observed orbital period distribution. The
available paths provide a natural division into systems characterized by long
orbital periods (> 60 d) and short orbital periods (< 30 d). Systems with
initial periods 1 - 2 d ultimately produce low mass He white dwarfs with short
orbital periods (< 1 d). For longer initial periods (> few days), early massive
Case B evolution produces CO white dwarfs with orbital periods < 20 d. Common
envelope evolution result in short period systems (P < 1 d) from unstable low
mass Case B evolution producing He white dwarfs, and from unstable Case C
evolution leading to CO white dwarfs. On the other hand, the long orbital
period group arises from stable low mass Case B evolution with initial periods
> few days producing low mass He white dwarfs and periods > 30 d, and from
stable Case C evolution producing CO white dwarfs. The lack of observed systems
between 23 and 56 days probably reflects the fact that for comparable initial
orbital periods (< few days) low mass Case B and early massive Case B evolution
lead to very discrepant final periods. We show in particular that the lower
limit (~ 23 d) cannot result from common-envelope evolution.
