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In Touch with Intonation
Christopher Mowatt
Luthier & Owner
Guitar-Tech Guitar Repair Services

String height affects takes us back to what intonation adjustments really do. When we adjust intonation we are compensating for the change in string length (and affects of increased tension) that occurs when a string is pressed against a fret. The further the string must go before it contacts the fret, the more the string changes length (the more the tension changes) the more compensation required to get that note to play in tune. String height is not only determined by the height of the saddle, but also by the amount of neck relief the instrument has. While I won’t go into relief adjustment here, it is one of those adjustments that any musician should become familiar with. We’ll save that for another article. By the way, I measure string height on electric guitars two frets from the last fret and on acoustic guitars I measure string height at the point where the neck joins the body; typically at fret 11 or 14 depending on the scale of the instrument. This is what I have found that gives me the information I need to set up the instrument. Many techs may measure at other points and that’s fine. In the end, its how the instrument plays that counts.

String slot wear is a very common problem and unless your guitar tech is familiar with the affect this has on intonation, it can be easily overlooked. Whenever I have a client that complains that chords are out of tune even though the guitar is in tune, it is a sure bet that the nut slots are the problem. What happens is that the front edge of the nut slot where the string makes contact (called the ‘witness point’) wears or chips away. This effectively makes the distance to the first fret longer so it makes the first fret note play sharp. To be certain the problem exists, measure and set the tuning of the open string and then measure the tuning at the first fret on the same string. If the fretted note is sharp while the open note is not, you have found the problem. A properly filed nut slot will not only bring the witness point to the front edge of the nut where it should be, it will slow down the speed the witness point will wear or chip away. Now, at the extreme end of nut problems, it is possible that the nut was actually mounted incorrectly or in the wrong position. Either of which requires a good eye to identify and in the case of a miss-located nut, special tools to correct.

Author’s biography: Chris Mowatt is a bass player who has been repairing fretted instruments for approximately 10 years with 6 more years experience building basses as Simpson- James Guitars. He holds a BS in Engineering and has over 20 years of process and manufacturing engineering experience in precision electro-mechanical systems.