Adam Clarke on Deuteronomy 24:6

The nether or the upper mill-stone - Small hand-mills which can be worked by a single person were formerly in use among the Jews, and are still used in many parts of the East. As therefore the day's meal was generally ground for each day, they keeping no stock beforehand, hence they were forbidden to take either of the stones to pledge, because in such a case the family must be without bread. On this account the text terms the millstone the man's life.