Planet Waves CT06 S.O.S. Guitar Tuner delivers a revolutionary design that pulses two out-of-phase LED light beams directly onto the vibrating string. Planet Waves CT06 S.O.S. Guitar Tuner indicates that the string is out of tune when two lights will visually dance on the string being tuned. As you approach in-tune status with the CT-06 SOS Guitar Tuner, the movement slows down and eventually stops when you are perfectly in tune. This revolutionary visual tuning system enables precision tuning in silent or noisy environments, without the need for audible sound or sensing of any kind.