The process of investigating empty space, begun previously in the work "Thesis and Antithesis", is taken up again in this series, albeit now with different assumptions.

Mateos now seeks to formally limit an internal space, turning it into an active space, always inscribed in a prism or cube that envelops it.

With this series, Mateos enters into a clearly constructivist investigation. They are works of great geometric rigour, close to rationalist and neoplasticist compositional approaches.

The texture of this series also reflects this change, as it has become smoother and more homogeneous, which refers us to a more aseptic finish in accordance with the rationalist postulates in which Mateos is moving.

One of these works, "Cubo cuarto", was chosen by Mateos to commemorate the death of his father, erecting it as a family burial place in the cemetery of Villavieja de Yeltes, where the sculptor rests today.