The Dura-Ace CN-7901 box says that it is...
The Dura-Ace CN-7901 box says that it is to be used for a double chain ring only. I have a tripple> Why won't it work.
Technically it will work though for the best shifting performance Shimano recommends using the 7901 chain with double cranksets only and 10 speed cassettes. Ten speed chains are narrower than 9 speed but do work on those drivetrains. You run in to more problems using wider chains on 10 speed systems.
Just be sure to install the chain with logos facing out as it is directional.