The introduction of the new Grover Locking Mini-Rotomatics^® gives you the ease of use provided by a standard tuner and the positive locking of our internal cam post. Simply insert the string through the string hole, pull to tension by hand and begin tuning. As you start to tune, only the locking cam in the string post turns. Once the string is locked, the string post begins to turn allowing you to bring the string to pitch. In less than one revolution of the string post, you will be in tune, to stay in tune. FYI: You absolutely need a lubricated nut with locking tuners. Gear ratio is 18 to 1. Hole size is 25/64" (.391") Model #406 Grover locking tuners are sold individually to easily cover every configuration possible. Pick and choose below depending on how the headstock is set up. $12 each