By 1963 Judd had established an essential vocabulary of forms — ‘stacks’, ‘boxes’ and ‘progressions’ — which preoccupied him for the next thirty years. Most of his output was in freestanding "specific objects" (the name of his seminal essay of 1965 published in Arts Yearbook 8, 1965), that used simple, often repeated forms to explore space and the use of space. Humble materials such as metals, industrial plywood, concrete and color-impregnated Plexiglas became staples of his career. Judd's first floor box structure was made in 1964, and his first floor box using Plexiglas followed one year later. Also by 1964, he began work on wall-mounted sculptures, and first developed the curved progression format of these works in 1964 as a development from his work on an untitled floor piece that set a hollow pipe into a solid wooden block. While Judd executed early works himself (in collaboration with his father, Roy Judd), in 1964 he began delegating fabrication to professional artisans and manufacturers (such as the industrial manufacturers Bernstein Brothers) based on his drawings. In 1965, Judd created his first stack, an arrangement of identical iron units stretching from floor to ceiling.