For all its success over nine decades, GAP&W – which had its offices in Burnham's Railway Exchange Building, where D.H. Burnham & Co. had maintained its offices – had no successor firm (it closed in 2006). The largest architectural firm in Chicago today and one of the major architectural firms worldwide – the world-renowned Skidmore, Owings & Merrill, aka SOM, which also has its headquarters in the Railway Exchange Building – is actually an offshoot of Burnham Brothers: Nathaniel A. Owings joined the practice in 1929 and left in 1934, and Louis Skidmore joined in 1930 and left in 1936. Skidmore and Owings started their own firm in 1936, and John Ogden Merrill joined them in 1939 to create SOM.