Old Patent Office, Washington DC
National Historic Landmark plaque marking the location of the Old Patent Office Building, Washington DC. This was the third public building constructed in Washington DC, after the White House and the U.S. Capitol. Formerly housed the Patent Office (1840-1932) and the Civil Service Commission (1932-1963), and is the current home of the National Portrait Gallery.
More information here: https://en.wikipedia.org/wiki/Old_Patent_Office_Building; http://npg.si.edu/about-us/history; https://web.archive.org/web/20070425215935/http://www.npg.si.edu/inform/chron.htm
Submitted by @yobigc