In his 1965 essay 'Specific Objects', Donald Judd argued that the borders of painting and sculpture had worn out, and a new kind of art was emerging — neither painting nor sculpture, but a 'specific object'. *Untitled (stack)* is the concrete realisation of that thesis.

The work consists of eleven identical elements: each a galvanised-steel box, 22.8 × 101.6 × 78.7 cm (later versions used copper, stainless steel, coloured plexiglass). The boxes are mounted on the wall at a right angle, with exactly 22.8 cm gaps between them, starting 22.8 cm above the floor — like the steps of a staircase that cannot be walked on. The interval between each element is as important as the element itself; the visual rhythm of the work is in the equal measure of object and space.

Judd was associated, alongside Carl Andre, Dan Flavin, Sol LeWitt, and Robert Morris, with the movement known as 'Minimalism' — he rejected the term, finding it shallow. His position was sharper: the 'content' of art is not a reference, a story, or a personal expression; art is the actual material, the actual geometry, and the actual space in which it is made.

The industrial mode of production is part of this position. Judd does not make the boxes by hand; he draws technical drawings and orders them from metal-working factories. There is no hand-labour of the artist in production — the precise opposite of Abstract Expressionism's thesis of 'the artist's body'. Each copy carries the same geometry; the concept of an 'original' becomes meaningless.

Many versions of the stacks were produced; MoMA, Tate, Guggenheim, and the Walker Art Center hold variants in different colours and materials. In 1991 Judd founded the Chinati Foundation in Marfa, Texas, a permanent centre exhibiting his work and that of his close contemporaries.