Vice President Spiro Agnew's resignation letter from October 10, 1973 and President Richard Nixon's resignation letter from August 9, 1974.
Title 3, Section 20 of the United States Code of Laws mandates that "an instrument in writing" must be delivered to the Secretary of State in order for the President or Vice President of the United States to resign from or refuse to accept their respective offices.
In the photos above, you can see where Secretary of State Henry Kissinger initialed and time-stamped the letters from Agnew and Nixon immediately upon receiving them, which formally accepted and triggered their resignations from office.














