Fret Calculation on the Classical Guitar

Without compensation at the saddle end, guitars tend to play sharper as you play higher up the fret board. This is mainly due to the increase in string tension as the string is fretted. Compensation is generally achieved by moving the saddle back the required amount. This varies according to string length, action height, tension etc but is generally around 1 to 2mm. It is usually achieved by moving the bridge (and saddle) rearwards from its nominal position when attaching the bridge to the top of the guitar. It is sometimes possible to make this adjustment later by moving the saddle within the bridge, but this is not a good alternative.

Without compensation at the nut end, guitars tend to play sharper in the lower frets, mainly the first five or so frets, and getting progressively worse as we approach the nut. One of the reasons for this is again string stretch as we depress the string to the fingerboard. The height of the nut is important here as it is generally set up fractionally higher than the fret height. The higher the nut, the harder the guitar to play and the worse the intonation problems in the lower frets and the greater the need for compensation. Intonation adjustment at the nut is sometimes called 'negative intonation'.

To solve this problem of sharpening in the lower frets it is necessary to "move" the nut forward toward the bridge, or effectively shorten the distance to the first fret. Each string should have its own amount of compensation and this is often done with high quality guitars. But keep in mind that if you do this it may need to be altered if you change to a different type of strings or alter the nut.

You may see some strange looking nuts around these days. Compensation at the nut is achievable in several ways. This picture shows compensation with a nut made specifically for that particular guitar and set of strings.

Keep in mind that this may need to change each time you change to a different string type or brand which had different physical properties of density, diameter, weight elasticity and so on.

Many luthiers still use a straight nut that has just been moved forward. This is a reasonable compromise if you are the type of player who changes brand and types of strings often.

Another point that is usually overlooked is the final sound of the guitar. With a correctly compensated nut and saddle and proper fret positioning, the guitar will not only play in tune but will also sound better and be easier to play. The sound will be less harsh and smoother.