Unifying partial descriptions of sets

A mattress includes a first bag forming a first cavity and being disposed within a second bag of substantially the same size. Each of the bags includes a pair of generally flat portions connected at a peripheral edge. Two separate continuous lap seams join the bags inwardly of their edge portions on opposite sides of the bags. These seams define with the edge portions of the bags a second cavity which is disposed around the first cavity. The first cavity can be filled with water to form a water mattress and the second cavity can be filled with air to form a peripheral air frame.