The Museum of Modern Art, MoMA Highlights, New York: The Museum of Modern Art, revised 2004, originally published 1999, p. 289

Sculpture must always face gravity, and the stack—one thing on top of another— is one of its basic ways of coping. The principle traditionally enforces a certain hierarchy, an upper object being not only usually different from a lower one but conceptually nobler, as when a statue stands on a pedestal. Yet in Judd's stack of galvanized–iron boxes, all of the units are identical; they are set on the wall and separated, so that none is subordinated to another's weight (and also so that the space around them plays a role in the work equivalent to theirs); and their regular climb—each of the twelve boxes is nine inches high, and they rest nine inches apart—suggests an infinitely extensible series, denying the possibility of a crowning summit. Judd's form of Minimalism

The field of Minimalist objects, however, is not an undifferentiated one—Judd also believed that sculpture needed what he called "polarization," some fundamental tension. Here, for example, the uniform boxes, their tops and undersides bare metal, suggest the industrial production line. Meanwhile their fronts and sides have a coat of green lacquer, which, although it is auto paint, is a little unevenly applied, and has a luscious glamour.