Originally Posted by
coasting
By the way, RX100 rear derailleur is also not compatible with 10 speed but is compatible with 9 speed. It simply does not have the adequate lateral movement to reach the tenth cog. I tried it with the shifter and also by hand pulling to maximum stretch.
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.