A certain amount of order was brought to Egyptian mythology and its myriad of gods by the priests themselves. The result of this was the creation of a number of enneads, groups of nine gods and goddesses, usually organized into families. The number nine was a sacred and symbolic number that could also stand for 'all' gods. This was because the Egyptians indicated plurals by using three, and nine was thus perceived as the plural of plurals. Therefore, 'nine' should not always be taken literally, the ennead of Thebes, for example, consists of fifteen gods. The most important ennead was that of Heliopolis, often called the Great Ennead. At its head was Atum, the creator and sun god. His children Shu and Tefnut gave birth to the earth god Geb and the sky goddess Nut, who in turn were the parents of Osiris, Isis, Nephthys and Seth. Occasionally Horus is added to them as a tenth god, sometimes as the fifth generation (child of Osiris and Isis) and sometimes as Nut's fifth child. In the Pyramid Texts of the Old Kingdom there is already mention of the five children of Nut. Centuries later, Plutarch relates how the sun god Helios (as the Greeks called the sun god Re) put a curse on the goddess Rhea (the Greek name for Nut), forbidding her to give birth to her children on any of the 360 days of the year. Hermes (Thoth in Egyptian) then added five days to the year (the five epagomenal days of the Egyptian calendar) and on these days Nut gave birth to her five children. In addition to the Great Ennead, there was also the Small Ennead, the gods of which varied with the times. The Small Ennead of Heliopolis included Isis's son Horus, Anubis, Maat and Thoth. Another Small Ennead included the Four Sons of Horus and the children of Khentekhtai.