The paper investigates definitions of k-tuples (k\(\geq 2)\) which are of the following form: \(=\{\{x_ 1\}\cup C_ 1,...,\{x_ k\}\cup C_ k\}\). A (rather artificial) definition adequate for any \(k\geq 2\) is given. However, if some natural constraints are satisfied then \(k=2\) and only the classical definitions of pairs by Hausdorff (1914), Wiener-Quine (1914, 1940), and Kuratowski (1921) are possible.