Which architectural innovation is Louis Sullivan associated with?

Louis Sullivan is prominently associated with the principle of functionalism in architecture, famously encapsulated in the phrase "form follows function." This idea emphasizes that the design of a building should be directly influenced by its intended use, leading to structures that are not only aesthetically pleasing but also practical. Sullivan is often referred to as the "father of skyscrapers" because of his pioneering designs that prioritized the functional requirements of tall buildings, resulting in innovative use of space, materials, and ornamental details that spoke to their purpose.

This approach marked a significant departure from the revivalist styles of the past, which often focused on traditional aesthetics rather than the actual function of the buildings. Sullivan's influence is seen in many of the skyscrapers built in Chicago and other cities, where he integrated steel frame construction with aesthetic concerns that aligned closely with the needs of modern urban life. This understanding of a building's design reflecting its function had a profound impact on future architectural trends, making Sullivan a key figure in the evolution of modern architecture.