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The American Institute of Architects Archives
Membership file contains a few letters about Institute matters, primarily Flagg's election to Fellowship, and an obituary clipping. Copies of pertinent pages from the AIA Convention Proceedings of 1923 and 1926 have also been scanned, to supplement the correspondence about Fellowship.
Avery Drawings & Archives Collection, Columbia University
Ernest Flagg Architectural Records and Papers, 1888-1972. For more information, http://library.columbia.edu/locations/avery/da.html
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