Intonation is a really important issue in guitar setup and maintenance. It's a key concern, period, if your playing is going to sound 'in tune', but if you're going to record with your band, or play live, then it's nothing short of vital.
Here's the lowdown on all things intonation - what it is, how to spot problems, and what to do about them! Take note!
Intonation refers to whether or not the guitar is in tune, anywhere along the fretboard. More on this with the next question...
A guitar is considered 'in tune' or 'tuned' if the open string notes are in tune - usually as recognized by some kind of tuning device/pedal/app etc.
Even a guitar with bad intonation can have its open strings tuned to the correct pitches and sound in tune when played.
The key with bad intonation is that the notes further along the fretboard i.e. The non-open strings, will not be quite the correct pitch they should be.
It's a common experience for guitarists to pick up a guitar and find that the further up the fretboard they play, the more out of tune it sounds, even if the instrument has just been correctly tuned. This is the biggest indicator of bad intonation.
You may well recognize an intonation problem simply by having the experience described in the previous point. If you're in the habit of soloing over backing tracks, for example, you'll spot the problem very easily, as you'll find your licks becoming increasingly out of tune as you work your way along the fretboard.
There's also a fairly standard intonation test you can do, and that is as follows:
Compare the two - by ear, with a chromatic tuner, or preferably both. If there's a difference (usually the fretted note being sharp) then you have an intonation problem.
The short version of the official advice on this is to take your guitar into a trusted store to leave it in their capable hands to be properly set up.
By all means, if you're good with home repairs and feel confident, it's absolutely possible to fix intonation yourself.
Like we mentioned before, you may check intonation by playing the notes on the 12th fret and they should match when playing the same string open. It works really well when you play a harmonic on the 12th and then fret the 12th. If the 12th fret is sharp, you want to move the saddle away from the fret, if the 12th fret is flat, then you must move the saddle closer to the fretboard.
One may begin adjusting intonation by adjusting the guitar's saddles. For example, a Stratocaster type guitar has saddles that can be adjusted so they move either towards the head of the guitar or towards the back of the guitar's body. These small adjustments will ultimately lead to the strings' length to either increase or decrease, which changes the pitch of the entire string.
However, if you're at all uncertain, visit a store and let them take care of it.