  A rook placement is a subset of a root system consisting of positive roots
with pairwise non-positive inner products. To each rook placement in a root
system one can assign the coadjoint orbit of the Borel subgroup of a reductive
algebraic group with this root system. Degenerations of such orbits induce a
natural partial order on the set of rook placements. We study combinatorial
structure of the set of rook placements in $A_{n-1}$ with respect to a slightly
different order and prove that this poset is graded.
