This is the Hipshot BT7 Bass Xtender: a replacement D-tuner machine head for the E-string of Mexican-made P-style and J-style electric bass guitars. Designed to offer fuss-free installation, this high-quality tuner comes complete with mounting materials and can be set up in a way that enables you to quickly lower the pitch of your E-string to a low D to play in Drop D tuning. Want to go even lower? No problem - the BT2 Bass Xtender can go as low as the low B. It also features a DoubleStop™, which gives you another tuning position between the high and low setting for a total of three tunings on one string.