Chains and cassettes  provide the means to transfer power to the rear wheel on a bike. Cassettes and chains are designed by "speeds" or the amount of gears they have. Cassettes come in 7, 8, 9 and 10 speeds. Beyond the speeds a cassette has, many cassette gearing choices are available to suit your riding ability. Chains frequently are broken down by 7 or 8 speed, 9 speed and 10 speed chains – but check the specifications because this does change by manufacturer.