Chicago's architecture is referred to as the Chicago School. The style is also known as Commercial style. In the history of architecture, the 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 parallel developments in European Modernism. A "Second Chicago School" later emerged in the 1940s and 1970s which pioneered new building technologies and structural systems such as the tube-frame structure.