Washington, DC

ZIP code 20231 is a special-purpose Washington, D.C., postal code historically associated with the U.S. Patent and Trademark Office, rather than a conventional neighborhood ZIP with clearly defined boundaries. Landmarks therefore cannot be reliably assigned to the code itself. Its best-known historical landmark association is the former Patent Office Building, now home to the Smithsonian American Art Museum and National Portrait Gallery; the building has a different current ZIP code.