Originally Posted by
JiveTurkey
10-speed cassettes and 9-speed cassettes are the same overall width. Actually, 10-speed is very slightly narrower, which is why it comes with a spacer for 8/9-speed hubs.
Well in that case RX100 rear derailleur isn't nine speed compatible either. I assumed it was becasue I could reach 9 of the 10 speed cassette.