  We make a detailed study of idempotent ideals that are traces of countably
generated projective right modules. We associate to such ideals an ascending
chain of finitely generated left ideals and, dually, a descending chain of
cofinitely generated right ideals.
  The study of the first sequence allows us to characterize trace ideals of
projective modules and to show that projective modules can always be lifted
modulo the trace ideal of a projective module. As a consequence we give some
new classification results of (countably generated) projective modules over
particular classes of semilocal rings. The study of the second sequence leads
us to consider projective modules over noetherian FCR-algebras; we make some
constructions of non-trivial projective modules showing that over such rings
the behavior of countably generated projective modules that are not direct sum
of finitely generated ones is, in general, quite complex.
