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 .