Louis Henry Sullivan (1856-1924), a granddaddy of American architecture, coined the phrase "form ever follows function." This simple statement was to become the creed of modernist designers and architects to follow, the line starting with Frank Lloyd Wright. Sullivan was working in the second half of the 19th century, a transformative time in architecture. The old way of building using load-bearing walls was being replaced with steel column-frame construction. The slate was blank, all constraints and rules of the past were blown away by the freedom of the delicate steel structure. Windows could be larger, buildings could be taller, slimmer, scrape the sky.
Looking at the pictured Prudential (Guaranty) building in Buffalo (that Sullivan designed in 1896 while in partnership with Dankmar Adler), you can see the 19th and 20th centuries colliding. The shape is modern, simple, divided into distinct 3 tiers. Each section has a different function: the bottom level is a public area for shops and foyers, the middle section is for offices and the top cornice is to house the machinery for the elevators. What differentiats Sullivan from his followers and holds him in the 19th century is his use of ornament. This building is dripping with terra cotta tiles in elaborate art nouveau patterns. Such unique use of ornament is a trademark of Sullivan's work. Drawing inspirtation from the traditional patterns of his Irish lineage, he used both cast steel and terra cotta to dress his work. And yet, these tiles are modular and manufactured on an industrial scale, referencing a hieroglyph laden temple but rooted in industrial America.