The PRL-9721-B0 is a set of staggered Post Ratio Locking Tuned Machine Heads for an electric guitar in a 6-in-line configuration. They have 3 different heights, contemporary buttons, a 2-pin casing and are finished in black. *Now includes InvisoMatch Standard Mounting Plates for easy installation. Fits most headstocks. Ratio - Tuned Machine Heads are the first and only machine head to incorporate the different string gauges into the tuning equation. Graph Tech balanced the individual gear ratios to each string dramatically simplifying the tuning process. No more overshooting the G string or twisting away on the high E string. With Ratio, each half key turn equals one semitone change on all strings! Quicker, more accurate tuning on every string.