Sullivan's design for the building was based on his belief that 'form follows function'. He and Adler divided the building into four zones. The basement was the mechanical and utility area. Since this level was below ground, it did not show on the face of the building. The next zone was the ground-floor zone which was the public areas for street-facing shops, public entrances and lobbies. The third zone was the office floors with identical office cells clustered around the central elevator shafts. The final zone was the terminating zone, consisting of elevator equipment, utilities and a few offices. The supporting steel structure of the building was embellished with terra cotta blocks. Different styles of block delineated the three visible zones of the building. Sullivan was quoted as saying, 'It must be every inch a proud and soaring thing, rising in sheer exultation that from bottom to top it is a unit without a single dissenting line.' The building was declared a National Historic Landmark in 1975.