The term 'Key Changer' refers to a device or tool used to change the key of a musical instrument. A CAPO, which is short for capotasto, is a type of key changer that is used on stringed instruments such as guitars and banjos. It is a clamp-like device that is attached to the neck of the instrument and can be used to raise the pitch of the strings, allowing the musician to play in a different key without having to re-tune the instrument.