This unique pick-sized 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 S.O.S. 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 on the money. A thumbwheel selects the right LED output for each string making the tuner specific to 6-string guitars. Since it relies strictly on visuals, the S.O.S is ideal for noisy and dim situations.