A vibration tuner which senses the sound through your instrument. You don't need to even plug your guitar in, even if it's an electric! Clip this beastie onto the headstock of guitar, bass or violin, and it shows which string you are tuning, a simulated analogue meter (LCD), and it turns green when you are in tune! Brilliant on stage, brilliant at home. Marvellous piece of kit! Can't recommend this highly enough!