Other divine groups were composed of deities with interrelated roles, or who together represented a region of the Egyptian mythological cosmos. There were sets of gods for the hours of the day and night and for each nome (province) of Egypt. Some of these groups contain a specific, symbolically important number of deities. Paired gods sometimes have similar roles, as do Isis and her sister Nephthys in their protection and support of Osiris. Other pairs stand for opposite but interrelated concepts that are part of a greater unity. Ra, who is dynamic and light-producing, and Osiris, who is static and shrouded in darkness, merge into a single god each night. Groups of three are linked with plurality in ancient Egyptian thought, and groups of four connote completeness. Rulers in the late New Kingdom promoted a particularly important group of three gods above all others: Amun, Ra, and Ptah. These deities stood for the plurality of all gods, as well as for their own cult centers (the major cities of Thebes, Heliopolis, and Memphis) and for many threefold sets of concepts in Egyptian religious thought. Sometimes Set, the patron god of the Nineteenth Dynasty kings and the embodiment of disorder within the world, was added to this group, which emphasized a single coherent vision of the pantheon.