Kluson modern Backlock 6L tuner with small button black. Highest possible tuning stability. The "Kluson Backlock System" allows you to run the string through the tuner shaft and fix it by tightening the knurled lock screw at the back of the tuner. A pin inside the shaft locks the string. The ingenious detail: when loosening the lock screw, it eventually ends up in a free spin so that the screw can never be lost.