The USB rechargeable version of the D'Addario Equinox headstock tuner precisely tunes guitars, basses and other stringed instruments utilizing vibrations rather than sound. Using a highly sensitive piezo sensor and large multi-color display, the USB Rechargeable Equinox headstock tuner allows you to tune easily and accurately in noisy and dimly lit environments where many other tuners fail. The tuner features a pivoting display and Micro USB port for charging. This is a pack of four.