One of the most influential figures in modern architecture, Frank Lloyd Wright initially worked under Louis Sullivan. While with Sullivan, he designed a number of houses on his own, known as his "bootlegged houses." The issue of those houses caused a break that sent Wright on his own in 1893. Over the next 20 years he became one of the best-known architects in the nation.Once his reputation was solidified, Wright began to reinforce the philosophical underpinnings of his innovative building style. He stated that the structural principles found in natural forms should guide modern American architecture. He praised the virtues of an organic architecture that would use reinforced concrete in configurations found in seashells and snails. He built skyscrapers the way trees were configured, with a central trunk deeply rooted in the ground and floors cantilevered from that trunk like branches.