Judd's rectilinear, freestanding works of the late 1960s helped to redefine the nature of sculpture. He used galvanized iron or aluminum, and Plexiglas in open sturctures which explored the relationships between space, scale and materials. These compositions were factory-made, fabricated by others and often determined according to mathematical progression. He upheld the idea that such primary structures were essentially different from Constructivisjm in that they achieved a wholeness through the repetition of identical untis in absolute summetry.