A method and an apparatus sort items, in particular of flat postal items, in two sorting processes. Two classes of items are sorted on the basis of a sorting feature such that a predetermined sequence of the feature values is maintained. In a first sorting process, a sorting system sorts the items in the first class separately from the items in the second class. In the process, the sorting system determines which possible feature values are actually assumed by at least one item in the second class. The sorting system generates a sequence of item sets. These item sets and the items in the second class are jointly sorted by a sorting installation. In the second sorting process, the sorting installation generates the predetermined feature value sequence of the items of both classes.