The capo, invented around the mid 1700s, is a small clamp-like device that essentially shortens the length of strings on fretted string instruments. Most commonly used by guitarists, a capos purpose is to clamp down on all the strings of your fretted instrument allowing the player to play in a higher pitch. Guitarists use a capo to change how high or how low a piece is played. Before the creation of a capo guitarist couldn’t really play certain songs in different pitches using the same fingerings.
A capo achieves its purpose by basically acting like a mobile nut to fretted stringed instruments. The nut is located at the beginning of the headstocks, and it holds the guitar strings in place, while canceling any vibrations passes the nut. For example, on a guitar, the standard tuning is E A D G B E from top to bottom on each open string, by using a capo we can clamp it somewhere on the fretboard and Essentially shorten it. This means we change the tuning of the open strings without actually tuning the instrument. Most people use it for its intended purpose, but the capo comes in many designs, materials, and shapes(especially over the last couple decades), it can also be used on any other fretted string instrument like the ukuleles, mandolins, mandolas, banjos, and bouzoukis.
As a guitarist and ukulele player, I have used a capo before. I personally don’t like using it as I don’t enjoy the feeling of having a shorter fretboard. The capo is also used a lot more in the pop genre, as capos can be used to eliminate the need to bar some chords. So as someone who plays a lot of classical music, no I don’t use it often nor do I like using it, but the device definitely works and is useful in many situations especially when trying to play the same piece/song in a different key.
Here’s a version of Pachbel’s Canon in D. In this video the player uses this technology to play the piece originally meant for piano and composed in D major. Using the capo on the second fret, the player is able to play the piece in D major. Without the capo he would have to either transcribe the piece for Dmj, or play it in C using the normal tuning. Essentially the capo is a powerful tool, because it allows a player to play the same notes/songs/piece in different keys and pitches