What is Intonation?
Intonation is when the guitar’s string is perfectly lined up with the frets. If the intonation is out you will find yourself more and more out of tune the further up the neck.
There are two reasons why the intonation on a guitar can go out. One is the bridge position might have moved as little as a tenth of a millimetre. The other is that the string(s) might be faulty or old.
An easy way to check your guitar is to hit a 12th fret harmonic and compare the pitch with a note fretted at the 12th fret. If the fretted note is sharp, the string needs to be lengthened by moving the bridge away from the neck. If the note is flat, then guitar’s bridge needs to be moved closer to the neck.