For any irrational number ξ let A(ξ) denote the set of all accumulation points of {z:z=q(qξ−p), p∈ℤ, q∈ℤ−{0}, p and q relatively prime}. In this paper the following theorem of Lekkerkerker is proved in a short and elementary way: The set A(ξ) is discrete and does not contain zero if and only if ξ is a quadratic irrational. The procedure used for this proof simultaneously takes care of a theorem of Ballieu.