The muster of the Levites included all the males from a month old and upwards, because they were to be sanctified to Jehovah in the place of the first-born; and it was at the age of a month that the latter were either to be given up or redeemed (comp. Num 3:40 and Num 3:43 with Num 18:16). In Num 3:17-20 the sons of Levi and their sons are enumerated, who were the founders of the mishpachoth among the Levites, as in Exo 6:16-19.