Best tuner!
Dont let the "guitar" in the subtitle fool you: this is an awesome tuner for any sort of string player, keyboardist, or wind player. I use it to tune harpsichords and organs in all sorts of temperaments. Its extremely fast and accurate. Love that you also plug in headphones and play a reference tone while it also listens through the microphone. A bargain of an app!
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PitchLab Guitar Tuner (FREE)