The position for a fret on a fingerboard for a stringed instrument having non-parallel strings of different scale lengths is determined by interpolating between, or extrapolating from, a fret position (R1, R2 etc.; G1, G2 etc.; Q1, Q2 etc.) on a first fret scale and a corresponding fret position (F1, F2 etc.) on a second fret scale (F-F; L-L).