A high quality nut file. A string seated in a properly filed nut will have the correct height and will also prevent extraneous string noise from the nut.0.028" / 0,71 mm width. A typical set for classical guitar would contain the following files:0.028", 0.032", 0.036", 0.042"A typical set for electric and acoustic guitars will depend on the string gauges. Always check the package for the string gauges and use the same file gauge accordingly.A great aid in filing nuts is the H-GB4 Hosco GrooBar. This will prevent you from filing the string slot too deep.