A postcard was a small piece of card — sometimes with an illustration or photograph on one side — that could be written on and posted without an envelope.
At some point prior to 1931, Trixie Trotter starred in a series of 'artistic' postcards. In one of these, she posed with a wall-mounted moose head, which led Arthur McFly to realize that she was Canadian rather than American and could not be the host for the Hill Valley Science Exposition.
When Emmett Brown and Marty McFly tried to deliver a subpoena to Arthur on June 13, 1931, Arthur told them to "put it in a postcard and send it", as Kid Tannen had told him not to leave his apartment.
When the Brown family ended up stranded in 1692, Doc constructed a remote control mail delivery time machine, which delivered a scratch and listen postcard to Marty in 1991.
