The cassette (the collection of sprockets on your rear wheel) determines the compatibility of almost all the other components of your bike’s drivetrain – from the chain, chainring and crank to the rear derailleur and shifter. In derailleur transmission systems, 7 to 12 sprockets are combined into one cassette, which is then slotted onto the freewheel body. The more sprockets you have (and the narrower the distances between them) the more gears the cassette offers. You’ll also need to consider the connection to the freewheel body of your hub. The most commonly used cassette/freewheel body standard is that shared by Shimano and SRAM. Campagnolo has its own unique standard.