A guitar capo is a simple device that a guitarist uses on the fretboard of a single-stringed instrument to shorten and bind the player's entire playing length to that of the guitar's neck length, thereby increasing the guitar's pitch. It's a favorite tool for acoustic guitar players of all ages, mandolin players in particular. You may also hear it used on a classical guitar, though its most frequent application is with the electric guitar. It may be called a "trunk," "head" or "neck" due to its placement on the sound hole of the guitar.
Guitar capos are designed to make it easier to change from one chord to another. They are easy to remove and replace if you want to change chords mid-song. Most capos attach to the bridge on the acoustic side of the guitar and run from there to the tailpiece on the electric side. A lot of capos these days also have a baroque style action that's similar to a barre that runs from end to end. Guitarists who play with open strings (and many acoustic guitar players) like capos with open strings that respond more readily to finger pressure rather than mechanical pressure of the capo engaging strings.
There are basically two types of capos: mechanical and optical. Mechanical capos turn when they are halfway opened and lock into place. They're useful for using with open-back chords, but it's not clear how they perform with strings with closed backs. Capos with an optical design, which are also sometimes called "traffic capos," work better with closed strings and seem to offer more control.
Capos are used widely in jazz guitar music. Early jazz guitarists often used them as substitutes for index and middle fingers. The earliest capos were much larger than the modern ones. Modern capos generally are much smaller than your typical guitar string, but there are some small capos that are still very useful for fingerpicking compositions. If you're looking for some inspiration about how to play jazz guitar, I would highly recommend checking out some free tab sheets of guitar music or downloading some free audio clips so you can see how a capo is used.
One common capo use is in "chord solo" style songs. In these songs, one guitarist will use a capo to play only a few notes, sometimes a few octaves, in a ascending pattern. Other capos are used to create melody in a song by letting the guitarist alternate various octaves with the open strings. A popular chord solo piece uses a minor pentatonic scale (a series of half-tones that are multi-seeded), played by the first guitarist in a group. With this type of partial capo, the second player's melody (the one the capo was playing) is heard only when the first guitarist's melody notes are passed over by the capo.
Another type of capo used in classical guitar is the arpeggio. This is also known as a half-tone capo, a flattened second fret, and a capo key signature. Some of the most famous arpeggios in history are: Oscar Peterson's OSS favorite "Mystery Train", Arturo Puente's "Comrade," and finally, George Harrison's classic "I'm a home on the hill". Most common for middle C, these capos allow the guitarist to play notes similar to a barre chord (the second fret of a G chord) without the fingers having to touch the strings. When used with the capo on the second fret, it gives a cool sound similar to an electric guitar tuner.
Many acoustic guitars have been made with capos which give them a bluesy or rock flavor. Even some classical guitarists have added a capo to their instrument. Many acoustic guitars come with the "stuck" or "fretted" notes, but the classical guitar capos come in handy for blues players who want that jangly sound.
Guitar capos come in many shapes and sizes to suit different styles of playing. In addition to the typical C, D, and A, many come in a variety of shapes to correspond with the different keys on a standard guitar. There are some models of acoustic guitars with two or three different key signatures. Some of them are double cut for the bass strings and others for the treble strings. And there are capos to match any guitar model, whether it is a nylon string acoustic, a steel string acoustic, or a classical model.