The “S” is actually a compromise between being named for his mother’s father and his father’s father, named Solomon Young and Anderson Shipp Truman, respectively. Still, Harry Truman usually signed his name with a period after the “S”, signifying that it was a middle initial. This was made even more confusing when he suggested in 1962 that newspapers should spell his name “Harry S Truman” without the period. However, for the federal government’s records and for the Harry S. Truman Library, his name is spelled with the period. According to the Chicago Manual of Style, that’s the correct way to do it.