Jamieson, Fausset and Brown on Deuteronomy 24:6

6. No man shall take the nether or the upper millstone to pledge--The "upper" stone being concave, covers the "nether" like a lid; and it has a small aperture, through which the corn is poured, as well as a handle by which it is turned. The propriety of the law was founded on the custom of grinding corn every morning for daily consumption. If either of the stones, therefore, which composed the handmill was wanting, a person would be deprived of his necessary provision.