Acoustic 3+3 Skeleton Button Nickel. The PRN-3411-N0 is a set of Ratio Open Back Tuned Machine Heads for an acoustic guitar that uses 3+3 tuners. It has a skeleton button and is finished in nickel. Ratio - Tuned Machine Heads are the first and only machine head to incorporate the different string gauges into the tuning equation. We balanced the individual gear ratios to each string dramatically simplified 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.