In part, we must look to the development of a theological system of thought in the Euphrates Valley to account for the superior position accorded to certain gods, and in part, political conditions and political changes afford an explanation for the union of certain deities into a family group.

So far, indeed, Amiaud is correct, that the relationship existing between the various deities, was as a rule expressed in terms applicable to human society.
The secondary position occupied, _e.g._, by Sin when compared with a god whose domain is the entire 'lower regions,' would be aptly expressed by calling the moon-god the eldest son of En-lil or Bel; and, similarly, a goddess like Bau would be called the daughter of Anu.

It is a mistake, however, to interpret the use of 'daughter' and 'son' literally.

Such terms are employed in all Semitic languages in a figurative sense, to indicate a dependent position of some sort.

Again, we have seen that the union of a number of cities or states under one head would be followed by a union of the deities proper to these cities or states.