Check the listing for details. Graph Tech 7-String Ratio 4+3 Electric Locking Tuning Machine Heads Blk 7 String. Color: Black, Condition: New. Listed at 159.60 USD. The Graph Tech 7-string Ratio 4+3 electric guitar locking machine heads incorporate different string gauges into the tuning equation. Graph Tech balanced the gear ratios to each string, simplifying the tuning process. With Ratio, a half turn is a semi-tone. Every string reacts the same to any tuning adjustments, delivering predictable, precise tuning on every string. The Graph Tech PRL-8521 is a right-handed set of locking Ratio calibrated machine heads for electric guitars that use 4 x 3 tuner configuration. It fits many Gibson, Epiphone and PRS-style guitars, and any other guitar with a 4+3 headstock.It has a two-pin casing that can be used as is, or with Graph Tech’s InvisoMatch mounting plates.