That’s the bar you see strapped around a guitar neck sometimes. I shifts the key while maintaining chord shapes. If a player uses it because they can only play three of four chords they need to spend some time learning other chords.
Thank you. I didn’t know that. Reading Joni Mitchell’s biography and I’m fascinated by how you can tune and retune your guitar for different sounds on different songs. And how some people just instinctively do it to get the sound that s in their heads.