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.