"My most nearly immortal contributions are an abbreviation and a typographical symbol.

I invented "iff" for "if and only if" --but I could never believe that I was really its first inventor. . . . . .

The symbol is definitely not my invention - it appeared in popular magazines (not mathematical ones) before I adopted it, but once, again, I seem to have introduced it into mathematics.  It is the symbol that sometimes looks like , and is used to indicate an end, usually the end of a proof.  It is most frequently called the 'tombstone', but at least one generous author referred to it as the 'halmos'."