When Harry Truman was born in 1884 his parents couldn't decide on a middle name, so they simply went with the letter S. Their goal was to honor both Truman's paternal grandfather (Shippe) and his maternal grandfather (Solomon).
Frustratingly for Truman, it actually led to some confusion during his lifetime — including when he took the oath of office to become the 33rd President of the United States.
From the notes to President Truman's autobiography:
Ever afterward Harry S. Truman had to explain that he possessed no middle name but that the S was not an initial and stood for two names. When on April 12, 1945 he took the presidential oath of office, Chief Justice Harlan F. Stone intoned, "I, Harry Shippe Truman..." and the new president replied, "I, Harry S. Truman..." Grammarians frequently refuse to place a period after Truman's middle initial. The holder of the initial customarily used a period, but omitted it when in a hurry.
Image: undated file photo of President Truman via Getty