How To Set Your Guitar’s Intonation - Difficulty: Easy
Take away – If the fretted note is lower than the harmonic, then the string is too long and should be shortened. Conversely, if the fretted note is higher than the harmonic, the string is too short and should be lengthened.
In order to sound good, your guitar must be able to play in tune. Intonation refers to the ability for a guitar to be properly tuned. There are several factors that can impact intonation including the straightness of the neck, the height of the strings, the condition of the nut and the length of the strings. For our purposes, we will assume that that your guitar is in good condition and that the neck is suitably straight. In that case, our focus will be on string length. For a guitar to be able to play in tune up and down the neck, each string needs to be at the proper length. Simply stated, the 12th fret must be the exact midpoint between the nut and the bridge.
Hold Finger gently on the 12th fret
Find the harmonic
One way to test a guitar’s intonation is to use a reliable electronic tuner to compare the pitch of each string fretted at the 12th fret, with a harmonic sounded at the 12th fret. A harmonic is achieved by very lightly touching your finger to the string (not pressing down at it) directly over the 12th fret, plucking it with your pick and quickly removing your left-hand finger from the string. It may take a little practice to get the harmonic to sound, but when you do, it will produce a clear ringing tone.
With a fresh set of strings on the guitar, compare the 12th fret harmonic of each string to the fretted note at the 12th fret. If the fretted note is lower than the harmonic, then the string is too long and should be shortened. Conversely, if the fretted note is higher than the harmonic, the string is too short and should be lengthened.
Adjusting the bridge
Most guitars have adjustable bridges with individual bridge pieces for each string. Some guitars will have only 3 bridge pieces, each one shared by two strings. Others still will have fixed, non-adjustable bridges. If your guitar has an adjustable bridge, you will most likely see a set screw behind the bridge for each string. After comparing the fretted note to the harmonic, if you have to change the 
string length, it’s best to de-tune the string creating less pressure on the bridge and making it easier to adjust. Be sure to only move the bridge a very short distance, then tune it back up and test it again. It may take a few tries to get it right, but the more you do this, the easier it will get.
Once you have your guitar properly intonated, you may not need to do it again for many months unless the instrument is jostled around a lot or if it undergoes severe temperature changes. This is an easy procedure for ensuring your guitar can be properly tuned. Good luck!
