A device for string instruments comprising a fretboard with a plurality of frets, arranged at a distance from each other along the lengthwise direction of the fretboard and running transversely across the fretboard. Over the fretboard, at right angles to and above the frets, are lengthwise tensioned strings (8), intended to be set in oscillation for tone generation, where the lengths of the oscillating parts of the strings are variable for variation of pitch by pressing the strings against selectable frets. At least some of the frets include a straight fret segment (18) located beneath at least one of the strings, which fret segment is displaced lengthwise along the strings in relation to other segments of the fret in question. In the manufacturing procedure for the device, values are measured for the position on the fretboard for contact points between string (8) and fret for correct intonation, whereupon corresponding fret slots with the determined said measured values for contact point positions on the fretboard, are individually milled across the fretboard, whereafter a prototype substance consisting of relatively soft material is pressed in to the slot, so a fret prototype with a corresponding shape is produced. A casting mould is made around said prototype, whereupon a fret is cast in the mould, which fret is thereafter inserted in the fret slot. Strings for musical instruments comprising a glass-metal material have also been described.