Chicago's architecture is famous throughout the world and one style is referred to as the Chicago School. Much of its early work is also known as Commercial style. In the history of architecture, the first Chicago School was a school of architects active in Chicago at the turn of the 20th century. They were among the first to promote the new technologies of steel-frame construction in commercial buildings, and developed a spatial aesthetic which co-evolved with, and then came to influence, parallel developments in European Modernism. A "Second Chicago School" with a modernist aesthetic emerged in the 1940s through 1970s, which pioneered new building technologies and structural systems, such as the tube-frame structure.