In general, the more speeds, the narrower the chain has to be. A 7 speed chain has a certain width of 7.3mm. An 8 speed is theoretically 7.1mm. But for those speeds, the chains are designated as 6/7/8 speed. When you get to 9 speed, you need a narrower chain, and 10 is even narrower. So if you've got a 9 speed cassette, you need a 9 speed chain. The 7/8 speed chains have reusable links, but above that the chain requires a single use master link. So if you are going to take your chain off to clean it, you will need to replace the master link, which costs between 2.00 and 4.50.
- for older bikes with a 5 or 6 speed freewheel, the 6/7/8 chains work perfectly.