  Let M to be a matroid defined on a finite set E. A subset L of E is locked in
M if L is 2-connected in M, the complement of L is 2-connected in the dual M*,
and min{r(L), r*(complement of L)} is greater than 1. In this paper, we prove
that the nontrivial facets of the bases polytope of M are described by the
locked subsets. We deduce that finding the maximum-weight basis of M is a
polynomial problem for matroids with a polynomial number of locked subsets.
This class of matroids is closed under 2-sums and contains uniform matroids.
