Judd often used Plexiglas in his work, since its colour was inherent in the material, and its transparency allowed a view into the interior of his structures. It meant that he no longer needed to make incisions to reveal the construction of the piece, as demonstrated in Untitled 1966, a sophisticated development of the earlier wooden floor boxes.
In 1965, Judd began to make vertical pieces – the so-called stacks – consisting of a number (usually 10, but variable according to the height of the room) of identical, projecting units hung one above the other on the wall. The intervening spaces have the same dimensions as the units, as does the space between the floor and the first unit, so that the boxes link the floor to the ceiling. These pieces are the first indication of his interest in integrating art with the architecture of the room. Judd used the standard stack form to experiment with different materials. In some the tops and bottoms are made of coloured Plexiglas which creates an astonishing effect. Colour glows in the intervening spaces and gives the stacks the appearance of gently gleaming columns.