An object associated with a set of two objects considered in a particular order. Thus the ordered pair <a,b> is not identical with the ordered pair <b,a>, just as the queue ‘a first and b next’ is not the queue ‘b first and a next’. In set theory, ordered pairs cannot be identified with the set of their members, since {a,b}={b,a}, which is exactly what we do not want. The task is to find sets with the property that <a,b> ≠a <b,a> if a ≠b, and with the property that if <a,b>=<u,v> then a=u and b=v. One solution is that of Kazimierz Kuratowski (1896–1980), that identifies the ordered pair <a,b> with the set {{a},{a,b}}. This is the definition in general use today. See also Cartesian product.