A capo (/ˈkeɪpoʊˌ kæ-ˌ kɑː-/ KAY-poh, KAH-; short for capodastro, capo tasto or capotasto [ˌkapoˈtasto], Italian for "head of fretboard") is a device a musician uses on the neck of a stringed (typically fretted) instrument to transpose and shorten the playable length of the strings—hence raising the pitch. It is a common tool for players of guitars, mandolins, mandolas, banjos, ukuleles and bouzoukis. The word derives from the Italian capotasto, which means the nut of a stringed instrument. The earliest known use of capotasto is by Giovanni Battista Doni who, in his Annotazioni of 1640, uses it to describe the nut of a viola da gamba. The first patented capo was designed by James Ashborn of Wolcottville, Connecticut year 1850.

Musicians commonly use a capo to raise the pitch of a fretted instrument so they can play in a different key using the same fingerings as playing open (i.e., without a capo). In effect, a capo uses a fret of an instrument to create a new nut at a higher note than the instrument's actual nut.

There are various capo designs, but most commercial capos consist of a rubber-covered bar that clamps to the instrument's neck in some way to hold down the strings. Capos come in different sizes and shapes for different instruments and fretboard curvatures. Factors that vary by type of capo are ease of use, size, degree of interference with the player's hands, and ability to hold down strings uniformly without affecting tuning. All types of capo should be applied after a fresh tuning by laying the barre, descending from above, and directly behind the fret, so that all of the strings have uniform position and pressure. If the strings are bent or mispositioned, the instrument sounds out of tune in the new key. Some types of capo can mar the neck of the guitar if applied incorrectly.

Song arrangements may cite capo position just as they cite alternative tunings. When referencing fingerings for a song that uses a capo, the player determines whether the chart references absolute finger positions, or positions relative to the capo. In tablature, for example, a note played on the fifth fret of an instrument capoed at the second fret can be listed as "5" (absolute) or "3" (relative to capo). Similarly, a D-shaped chord can be referred to as "D" (based on the shape relative to the capo), or E (based on the absolute audible chord produced). Neither method strongly prevails over the other. For this reason, the phrase "chord-shape" is commonly used to clarify that the fingering shape and not the audible pitch is being referred to.

With this concept in mind, if two players want to play a chord progression in a more interesting way, one can play first position chord-shapes with no capo, while the second player places the capo further up the fretboard and plays different voicings of the same chords. This creates a fuller sound than two guitars playing in unison. For example, if they play a simple I IV V chord progression together in E the first guitarist plays E A B7 while the second plays the same progression capoed at the fourth fret using C F G7 chord-shapes.

Playing with a capo creates the same musical effect as retuning all strings up the same number of steps. However, using a capo only affects the open note of each string. Every other fret remains unaffected (e.g., the seventh fret of an E-string still plays a B note for any capo position at or below the seventh fret), and thus a performer does not need to adjust for or relearn the entire fretboard as they might with retuning. The scale length of the strings of an instrument affects the timbre of the strings, and thus the use of a capo may alter the tone of the instrument.

Musicians also use capos to bring a guitar tuned below standard up to standard tuning. Manufacturers sometimes recommend tuning a twelve-string guitar a whole-step or more below standard to offset the additional stress of the additional strings. A capo can raise it to standard tuning. However, through improved manufacturing techniques, many modern 12-strings are tuned to standard pitch.

Some guitar styles—such as flamenco, Irish traditional music, and British and American folk music—frequently use a capo. Others—such as classical and jazz—rarely use a capo. Many rock and roll musicians who are influenced by folk and blues, such as Richard Thompson, Keith Richards, Ry Cooder, Ian Anderson, Steve Earle, George Harrison, Tom Petty, Bob Dylan, Noel Gallagher, Steve Rothery, Johnny Marr, and others also use the capo. In many cases, they have extended its use past the traditional purpose of changing the key, and broken new ground, employing it in new ways.^{[citation needed]}