A capo (short for capo tasto) is a device used for shortening the strings, and hence raising the pitch, of a stringed instrument such as a guitar or a banjo. There are several different styles of capo available, utilising a range of mechanisms, but most use a rubber-covered bar to hold down the strings, fastened with a strip of elastic or nylon, a cam-operated metal clamp, or another device.
The use of a capo is considered by some people to be a crutch for technically inferior players. While it can be used for this purpose (for example, allowing a novice guitarist to play chords in the relatively difficult key of Ab by playing the much simpler chord shapes for the key of G), it also facilitates making use of the instrument's natural qualities in certain keys and allows for the use of techniques and sounds that would otherwise be unavailable.
Because of the different techniques and chord voicings available in different keys, the same piece may sound very different played in D or played in C with a capo at the 2nd fret (at the same actual pitch). Additionally, the timbre of the strings changes as the scale length is shortened, suggesting other short-scaled stringed instruments such as the mandolin. Therefore the use of a capo is as much a matter of artistic expression as of technical expediency.
The use of a capo also obviates the need to learn a song in several different keys if accompanying singers who sing at different pitches.
For guitar playing, some styles such as flamenco and British/American folk music make extensive use of the capo, while it is used very rarely if at all in other styles such as classical and jazz playing.