 It is the oldest surviving skyscraper in the world to use a purely skeletal supporting structure.The building was listed on the National Register of Historic Places on March 16, 1976, and designated a Chicago Landmark on July 7, 1978.The distinctive bow windows provide light into the building's interior spaces, and the combination of a granite facade for the lower floors and brick facade for the upper stories helps lighten the load placed on the internal steel framework.