Guitar capos change the key of your instrument by placing pressure on the fingerboard around a particular fret, a utility which has seen them at the forefront of traditional and contemporary music for hundreds of years. They are especially useful for singer-guitarists, as they allow them to transpose songs to better suit their vocal range.
Guitar capos are derived from the capotasto, a device used to change the key of viola de gambas as early as 1640. The name in Italian means 'head' (capo) 'key' (tasto). Originally simple straps, they have evolved into reliable screw and spring mechanisms. It’s worth noting that nylon- and steel-string guitars have different-shaped fingerboards, and thus require specific capos.