The unique pick-sized Planet Waves SOS Strobe Tuner uses Planet Wave's revolutionary Strobe-On-String technology. If you've set a turntable's pitch via a stroboscopic control or use harmonic comparative tuning at multiple fret points to tune a guitar, you will immediately get the concept. Barely bigger than a guitar pick, the SOS Tuner emits 2 out-of-phase LED light beams onto the vibrating string. The 2 beams dance on the string when it's out of tune, slow down as you approach pitch, and stop when you're in tune. A thumbwheel selects the correct LED output for each string, making the Planet Waves SOS Tuner specific to 6-string guitars. Since it relies strictly on visuals, the SOS Tuner is ideal for noisy and dim situations. NOTE: The Planet Waves SOS Strobe Tuner only works with 6-string guitars in standard tuning. The tuner's lights may be difficult to see on some clear nylon classical guitar treble strings.