Gotoh Standard Machine Heads - 6 in a line - SD91 -05M - Black Mounting hardware included. Through its use of the ideal materials selected for each component part you also obtain a far superior sound The SD series uses a torque balancer to ensure steady rotation of the tuner button giving high quality sound performance as well as great functionality The SD series solves these problems by combining "Rock-solid" technology and the post collar or: "Self-Center Guide", found on enclosed gears in the base plate of the gear allowing the post to perfectly align itself with the bushing which attaches the gear to the headstock Kluson type machine heads are criticized for not having the same smooth performance, efficiency and durability that sealed gear tuners have