The revolutionary Planet Waves S.O.S. Guitar Tuner pulses two out-of-phase LED light beams directly onto the vibrating string. When the string is out of tune, the two lights will visually dance on the string being tuned. As you approach in-tune status, the movement slows and then stops when the string is perfectly in tune. This revolutionary visual tuning system enables precision tuning in dark and noisy environments, without the need for audible sound or sensing of any kind. Ergonomic pick-size design.