Numbers 3:15
Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shalt thou number them.
Number the children of Levi,.... Who were left out of the general muster of the children of Israel, but now were to be numbered alone, and in a different manner, and for, a different purpose, the Lord having a special regard auto, them, and special service for them:
after the house of their fathers, by their families; into which the tribe was divided; house seems to be put for "houses", which were principal ones; and these were divided into families, which branched from them, and according to these, denominated from their fathers, and not their, mothers, were they to be numbered; for as the Jewish writers often say, a mother's family is no family; wherefore, if a Levite woman married into any other tribe, as she might, her, descendants were not taken into this accounts only such whose fathers were Levites, see Numbers 1:2;
every male from a month old and upwards shalt thou number them; the reason of this was, because the firstborn, for whom they were to be exchanged, were at a month old claimed by the Lord as his, and to be redeemed; and as this numbering was on, another account than, that of the children of Israel, who were numbered from twenty years of age and upwards, that they might on occasion be called out to war, from which the Levites were exempted, and the numbering of them was for the service of the sanctuary; so from their youth they were to be brought up and trained for this, that they might be fit for it, and enter upon it at a proper age.