On a bicycle, the cogset is the set of multiple rear sprockets that attaches to the hub on the rear wheel. A cogset works with a rear derailleur to give multiple gear ratios to the rider. Cogsets come in two varieties, cassettes or freewheels, of which cassettes are a newer development. Although cassettes and freewheels perform the same function and look almost the same when installed, they have important mechanical differences and are not interchangeable. A freewheel (also known as a block or cluster) consists of the rear set of sprockets and a ratcheting mechanism in a single assembly that mounts on a threaded hub. Traditional rear hubs have a standardized right-hand thread (1.375 x 24 TPI) onto which a standard freewheel is screwed. This allows different brands of freewheels to be mounted on different brands of hubs. Cassettes are distinguished from freewheels in that a cassette has a series of straight splines that form the mechanical connection between the sprockets and the cassette compatible hub, called a freehub, which contains the ratcheting mechanism. The entire cassette is held on the hub by means of a threaded lockring.