There's a whole bunch of 'magic' tricks that rely on that property of nine (or, for the really obscure ones, the digit N in numbers of base (N+1)) In fact, if I remember correctly, Martin Gardner had a chapter on that in his book 'Mathematical Puzzles and Diversions' (Or perhaps his other book 'More Mathematical ...') One concerned a sort of telephone dial with weird symbols, but they were just flummery to hide the numerical basics.