Originally Posted by
HillRider
Each successive increase in cog numbers required a thinner chain to maintain adequate clearance between increasingly narrow cog gaps. So, within reason, chains are backwards compatible, i.e a 10-speed chain will work on an 8 or 9-speed cassette but not the reverse since the wider chain won't fit well in the narrow cog gaps.
HillRider this is valid to a point but in some cases the chain is too thin to make the jump to the next lower cog that is dictated by the brifter and it can become a real mess trying to compensate for this while riding the bike or even trying to set it up on a stand.