Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shall you number them.
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George Leo Haydock
Month; at which time the first-born were to be redeemed, chap xviii. 16. (Menochius)
If only those of 20 years of age had been counted, they could not have amounted to nearly an equal number with the first-born of all the other tribes. (Calmet)