Guitar intonation refers to defining the correct length of each string in relation to the guitar neck.
We know that the guitar neck has a fixed scale, determined by the sequence of frets. This fretboard is what we use to produce individual notes in the guitar, resulting in one of the 12 notes of the chromatic scale.
The Problem of Intonation
It turns out that the fretboard can only be in tune if the length of the strings is correct. For example, suppose that the string is too long. Then, the locations in the fret board will not be completely in tune, specially as you go down the guitar neck.
To understand why, just imagine how the fretboard should be when the length of the string increases. To keep the proportions correct, the distance between frets in the fretboard should also increase in size.
A similar problem occurs when the string length is too short. The spacing of the frets will also be incorrect. These differences result in a guitar that is slightly out of tune across the fretboard, which can become a problem as you play the instrument.
To fix these intonation issues, then, we need to make sure that the length of the string is correct, in the right proportion compared to the fretboard. To guarantee this, you need to make adjustments to the bridge as described in the next sessions.
Checking for Intonation Issues
First, we need to determine if the guitar is suffering from intonation issues.
To determine if you guitar has the right intonation, you need to check if the strings are in tune, specially when reaching the higher frets (starting in the 12th fret).
To do this, you need to use a digital tuner and press each string on the 12th fret. Make sure that the open string is in tune, and that it is also in tune on the 12th fret.
If there are differences in intonation, that probably means that you need to adjust the string.
Fixing Intonation Problems
To solve intonation problems, we need to guarantee that the string length is proportional to the guitar neck. This is done differently in each guitar model.
A few guitars have a screw that change the position of the bridge for individual strings. This method is used, for example, in the Fender Stratocaster.
On the other hand, a few models require you to move the whole bridge left or right to change intonation.
Video Explanation
This is a topic that is easy to show than to explain using words.
The following video shows a good visual explanation of how the intonation process works.
The next video shows the necessary steps to start the intonation process. Then, you can see how easy it is to change the information using the screw system. Notice how it gets easier when you prepare the instrument and use the proper tools.
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