On Representatives of Subsets

Let a set S of mn things be divided into m classes of n things each in two distinct ways, (a) and (b); so that there are m (a)-classes and m (b)-classes. Then it is always possible to find a set R of m things of S which is at one and the same time a C.S.R. (= complete system of rcpresentatives) for the (a)-classes, and also a C.S.R. for the (b)-classes.