The guitar capo.

The original capo was designed in the Renaissance and used by lute players.
It's original Italian word is "capotasto" The first known capo dates back to the 16th century where it was used by Giovanni Battista Doni (bap. 13 March 1595 – 1 December 1647) uses it to describe the nut of a viola da gamba.[1] The first patented capo was designed by James Ashborn of Wolcottville, Connecticut year 1850.[2] What is it used for. Let's say for example you only know the 8 open chords and you want to play a "C" chord but higher up th neck because the open chord "C" is too low in pitch for you or another to sing a song. A good example is Tom Petty's song "Free Fallin" which is in the key of "F". Let's say you want to use the "D" chord istead because it's easier to play thr sus2 and sus4 chords with an open ringing sound. Simply place the capo on the ? fret and there you have it.