  In choosing a family of histories for a system, it is often convenient to
choose a succession of locations in phase space, rather than configuration
space, for comparison to classical histories. Although there are no good
projections onto phase space, several approximate projections have been used in
the past; three of these are examined in this paper. Expressions are derived
for the probabilities of histories containing arbitrary numbers of projections
onto phase space, and the conditions for the decoherence of these histories are
studied.
