Search a song and the tuner sets itself up for it — Nutshell drops you into Eb standard, Chop Suey! into drop C. The tunings are as recorded, per tab consensus, but bands re-tune live and a few are genuinely argued over, so trust your ear over the list. Anything missing, + Add puts it in permanently on this device.

Play one string open. The wheel drifts clockwise when you're sharp, counter-clockwise when you're flat, and stops dead when you're in. Tap a string to lock the target — do that when you're a whole step out, or auto-detect will keep snapping to the nearest string instead of the one you're heading for. Ref plays the locked pitch.

Under the hood: YIN pitch detection — cumulative mean normalized difference over a 2048-sample window, parabolic interpolation for sub-sample period, 50–1200 Hz, 5-frame median filter. Browser mic processing (AGC, noise suppression, echo cancellation) is switched off; it pumps amplitude and eats the fundamental, which wrecks a tuner. Resolution is sub-cent, where an FFT at this window size would give you ±11 Hz bins — useless on a low B.

Desktop: space toggles the mic, 1–7 lock a string. Keep the phone a hand's width from the speaker cone or pickup; the level bar wants to sit around a third full.