According to the interwebz regardless of the outer width of the chain (more "speed", narrower chain), these are the inner dimensions:
Multi speed chains, from 5 to 8 have inner width of 3/32″ (2.38 mm).
Multi speed chains from 9 to 12 speeds have inner width of 11/128″
(2.18 mm).
In line with this, I'm using a 9 speed octalink ultegra crankset on a 10sp group, and also running 11sp chains on my other 10sp systems without problems (actually it quieted up my 1x10 gravel's drivetrain)
In terms of outer diameter, a 10sp chain is 1.22mm narrower, that -combined with the chainline thing- would make me question if it can drop between rings.
(Also, imagine measuring things as 11/128th of an inch. Just lol)
According to the interwebz regardless of the outer width of the chain (more "speed", narrower chain), these are the inner dimensions:
In line with this, I'm using a 9 speed octalink ultegra crankset on a 10sp group, and also running 11sp chains on my other 10sp systems without problems (actually it quieted up my 1x10 gravel's drivetrain)
According to Sheldon Brown 9speed on 8speed chainring should be workable, but I havent tried. He also mentions that with 9sp the small chainring's design positions the actual chainline ever so slightly closer to the large ring.
In terms of outer diameter, a 10sp chain is 1.22mm narrower, that -combined with the chainline thing- would make me question if it can drop between rings.
(Also, imagine measuring things as 11/128th of an inch. Just lol)