What does the "S" stand for in Harry S. Truman's name?

Answer: Nothing, it's just an initial. 

Harry S. Truman actually had no middle name. His parents gave him the middle initial S to honor and please his grandfathers, Anderson Shipp Truman and Solomon Young. Since the S did not stand for a name, Harry didn’t use a period after it for most of his life. Soon after he was elected president, the editors of the Chicago Style Manual informed Truman that omitting a period after his middle initial was improper grammar and a bad example for America’s youth. From that moment on, the 33rd President signed his name Harry S. Truman or put all the letters in his name together as in HarrySTruman.

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