Washington City Post Office Historic Lobby, National Postal Museum

For 72 years this historic building was the main post office of the District of Columbia. Since 1993 this building has housed the Smithsonian's National Postal Museum. Before that, it was the Washington City Post Office, built next to Union Station to speed the movement of incoming and outgoing mail. Picture the lobby filled with people buying stamps, checking mailboxes, and socializing—in a time when mail was the primary means of communication.