  Rough sets induced by quasiorders appear in several constructions using
binary relations in computer science. In this paper, a structural
characterisation of rough sets induced by quasiorders is given. These rough
sets form Nelson algebras defined on algebraic lattices. We prove that any
Nelson algebra can be represented as a subalgebra of an algebra defined on
rough sets induced by a suitable quasiorder. We also show that Monteiro spaces,
rough sets induced by quasiorders and Nelson algebras defined on $\rm
T_0$-spaces that are Alexandrov topologies can be considered as equivalent
structures, because they determine each other up to isomorphism.
