CASSETTE and CHAIN
Bike Cassette is a combination of cogs and free wheel. You can have cassette from 6 to 11 cogs or simply a single cog when it comes for fixie or BMX bikes. There are different type of cogs and rules how they can be combined based on the number of tooth for instance 11 to 32 or 11 to 34 etc.
The bike chain should fit the cassette of one bike. We can have chains for 9 gears or 8 gears or combined ones which covers 6,7 or 8 gears. The right choice of your chain depends on the number of gears you have and thus the distance between the cogs.
Tip: Normally a chain should be replaced at c.a. 2000-3000 km and with everz 4th chain replacment the cassette should also be replaced with a new one. Please consult your near bike shop for more information if zou are not sure how and whether the chain shoudl be replaced.