"form ever follows function"
One of Sullivan's most notable contributions was the creation of a form appropriate to the tall commercial office building. Rather than stressing the horizontal layers of each story, he emphasized the vertical rise of these buildings. Verticality was made possible by steel frame construction and the use of light materials such as terra cotta, which had a malleability appropriate for carrying out his ornament. 
The Carson Pirie Scott Store, designed by Louis Sullivan in 1899, marked the high point of the functional tradition in the Chicago school and is the foremost American example of the transformation of utility and structure into powerful architecture. Ibidem