A capo (/ˈkeɪ.poʊ/ or /ˈkæ.poʊ/; short for capo d'astro, capo tasto or capotasto [kapoˈtasto], Italian for "head of fretboard"; Spanish, capodastro [ka.po'ðas.tɾo]) is a device used on the neck of a stringed (typically fretted) instrument to shorten the playable length of the strings, hence raising the pitch. It is frequently used on guitars, mandolins, and banjos.
The word derives from the Italian "capotasto" which means the nut of a stringed instrument. The earliest known use of the term "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, USA.
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 several different 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 fret board curvatures.
The most relevant mechanical 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 will sound out of tune in the new key. Some types of capo can mar the neck of the guitar if applied incorrectly.
Musicians use capos on many stringed instruments: guitars, mandolins, mandolas, banjos, bouzoukis - virtually any instrument that has strings suspended over a fretted fingerboard. Capos exist for square-necked resonator guitars, some of which do not contact the neck, but clamp above and below the strings.
Back-story: When I first starting playing a string instrument, I had no idea what a capo was. I learned three chords and was fine until someone decided to change the key. I was stuck. I had to learn a bunch more chords to stay in the same tuning.
Somebody taught me how to play a barre chord (which is basically a capo but using your fingers). I struggled because it is hard to keep the pressure down on all the strings. Now I could slide up and down the neck to find any chord necessary to play a song.
Then somebody showed me a capo. A steel bar wrapped in rubber connected to an elastic band. The band could unlock at one end and wrap about the back of the neck to hook again. The barre was placed in between frets and could be adjusted until all the strings matched pitch.
The 12-string had even more pressure so the capo had two elastic straps.
It was a wonderful system until the elasticity loosened.
Then there were mechanical metal barre that would slide over the next and lock in place. Unfortunately not all necks are the same and some didn’t fit the width.
Now there are dozens, maybe hundreds of variations that are spring loaded. Easy to move and take off, the capo has become as important a tool for a string player as a pick and a tuner.
After years of watching different techniques and styles, I learned to transpose. It was faster on the fly playing and kept the sound different. If someone wanted to play the chord C major, I know half dozen variations that don’t need a capo.
The capo is still in my toolbox and is a great tool to do one job and do it very well.