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Amit Newton
Doha
The nut (near the headstock) and the saddle (in the bridge) are the two points that determine the effective vibrating length of a guitar string. It is also true that the fretboard distance from the nut to the 12th fret, and from the 12th fret to the saddle, are equal, or, at least, should be. If equal, then a string stretched over such a fretboard, would sound its exact octave when fretted and played at the 12th fret, as compared to it being played untouched (open).
If that is the case over all strings, the guitar is said to be correctly intonated. If these distances are not equal, it gives rise to intonation issues. This affects how chords (a combination of notes) sound and even how individual notes sound in relation to each other.
Generally speaking, brand new guitars don't show intonation issues. If a brand new instrument is showing such problems, do not buy the instrument. In a used instrument, such issues may crop up after a few years of playing, and the factors responsible for it are varied, including, scale length, string thickness, health/nature of frets, compensation of saddle and nut, neck relief and even individual playing style.
String thickness
There are two basic facts about strings that bear on playability: One, the longer a string is (ie longer scale length) the more tension is required to bring it to pitch. Two, the thicker a string is, the stiffer it is (more tension).
Excess tension limits the string's ability to vibrate freely. A stiff string is not only difficult to play, it is also hard to keep in tune, as it causes the tuning machines to work harder than necessary. Keep in mind also that the angle of the string over the nut, and the degree to which a string moves at an angle from the nut to the tuning post, will increase tension in the string.
Health/nature of frets
Guitar frets come in all sizes and types. But jumbo frets that sit really proud on the fretboard, demand greater pressure to push a string down to the fretboard (referred to as fretting a string). Besides the physical energy expended in the act, it also turns out that all that force causes the note to go sharp, throwing the intonation out of sync.
Also, worn out, damaged frets, or, those that were filed and left uncrowned (see illustration) are a problem, affecting intonation big time.
Compensation of saddle and nut
Given their different thicknesses, individual strings need to adjusted to play in tune and in unison. This is done by 'compensating' for their thicknesses at the saddle and the nut (see picture).
Installing an extra wide saddle piece so that the breakaway point for each string can be individually positioned (compensated) along the length of the saddle, helps in a big way.
Nut compensation is now a common practice. It is done by moving the breakaway point of the nut (which should be at the very edge) closer to the saddle a short distance. Nut compensation helps minimise the difference between notes played near the nut (which tend to be a bit sharp) and those between the 4th and 12th fret, which can run a little flat. The action at the nut (that is, between the nut and the first fret) will also have a bearing on the compensation required at the nut. Generally, the lower the action can go at the nut, without generating string buzz, the easier it will be to intonate the guitar and the less nut compensation will be required.
Neck relief
The more the relief in the neck, higher will be the action, and thus, greater the force required to fret a string. That large amount of force tends to add tension to the string, which in turn makes the fretted note go sharp. For correct intonation, relief in the neck should be just enough so that it does not cause string buzz.
NEXT TIME: All about fretboards
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