Deraileurs

There's no point in having gears if you can't shift when you need to. Deraileurs, or 'mechs', take up that responsibility, moving the chain from one chainring or cog to another, to facilitate lowering or raising the gear. Most bikes will have a front and a rear deraileur, and they're designed to work seamlessly within a groupset. Mixing and matching your deraileurs, cranksets and shifters is a slightly more tricky prospect, but some people fine tune their bikes this way for more precise shifting or for a more robust set-up. Make sure you're marrying the right deraileur to your gears before you start riding though; you'll only shift your chain into the spokes of your rear wheel once, believe me!