  In this paper, the ordered set of rough sets determined by a quasiorder
relation $R$ is investigated. We prove that this ordered set is a complete,
completely distributive lattice. We show that on this lattice can be defined
three different kinds of complementation operations, and we describe its
completely join-irreducible elements. We also characterize the case in which
this lattice is a Stone lattice. Our results generalize some results of J.
Pomykala and J. A. Pomykala (1988) and M. Gehrke and E. Walker (1992) in case
$R$ is an equivalence.
