Carlo Aymonino brought Aldo Rossi into the project to design part of the northern extension. By early 1968, the scheme had evolved into the five building complex as it basically exists today. Aymonino was responsible for buildings A1, A2, B, & C, and Rossi designed the fifth building "D" that extends to the north, paralleling building "B".
Aymonino's slabs are complex and typologically diverse in the extreme. Rossi's building, by contrast, is entirely repetitive and white. Referred to by one writer as la lama bianca, it was a reference, according to Rossi, to the galleried ballatoio housing of Milan of the 1920's. But this elementary structuralist slab makes reference as well, to Rational Architecture of the 20's and 30's, and to the post war popular housing built near the Gallaratese in the INA Casa complex along Via Harrar between 1953 and 1955. Almost 200 meters long, this block contains two and three floors of gallery-access flats raised on a high base containing a public arcade. The buildings of Aymonino, which originate and radiate from a common source--the nodal amphitheater--are rendered in a dark brown color and share similar details, red window frames, glass block, and balconies, and seem derived from the same complex cross section. Rossi's slab on the other hand, is detached from the other buildings and, in contrast to its neighbors, is derived from a most elementary section; a continuous, constant, repetitive wall that has a curious under-scaled, diminutive, toy-like quality because of the use of a narrow structural grid. Aymonino and Rossi worked together for ten years and between them produced seminal research about architecture and the history of the city, and, indeed, were the proponents of a revived interest in design as the logical extension of historic precedents. When they build together for the first time, however, there is an enormous conflict of style, two diametrically opposite attitudes about building.
Text source: Housing Prototype