For fixed, non-adjustable bridges on Bouzouki, Mandola, and Mandolin. This system allows you to add a pickup to your instrument without having to change the bridge or modify anything . The BM-1 Pro, with volume control, was designed with a lengthened sensor element to enable it work properly on the bridges of Bouzouki's, Mandola's, and fixed (non-adjustable) bridges of Mandolins.