The Chicago Auditorium Building (1887-1889) is considered as the main work of Adler & Sullivan. It is a mixed program which houses a monumental auditorium (4300 places) with excellent acoustics, offices and a hotel in the back. This technical feat was allowed thanks to a very thin metal structure freeing at the maximum the interior space. Frontages present an original rustic stones base, contrasting with smooth stones of current stages. With <> dominant lines, the small tower appears almost anecdotic ! 1946 the auditorium being too vast, the building was repurchased by the Roosevelt University.