Números 18

Números 18:20

For all this people had twelve tribes after the number of the twelve sons of Jacob. What we call tribes are as it were distinct houses and congregations of people. This people, I say, had twelve tribes, out of which twelve tribes one tribe was Judah, out of which came the kings. There was another tribe, Levi, out of which came the priests. To the priests serving the temple no land was allotted. So it was necessary that among twelve tribes all the land of promise should be shared. Having separated one tribe of higher dignity, the tribe of Levi, which was of the priests, there would have remained eleven, unless by the adoption of the two sons of Joseph the number twelve were completed. — EXPLANATION OF THE PSALMS 76:1