Have you noticed that your G string often sounds slightly out of tune? There are many factors that could be causing this, so let's go through them...
Why Always Sharp G String
Headstock Design
On guitar headstocks, particularly those with 3R+3L tuning machine placement, the D and G strings are slightly longer and at a sharper angle than the other strings. This can increase the tension on the guitar strings, making the G string seem slightly out of tune.
Nut Slot
The friction between a string and the nut slot can cause a majority of tuning problems. Every time you bend a string, it moves in the nut slot, causing the tension exerted on the string to change, which can also alter its pitch. To minimize this, you can lubricate each slot by adding pencil lead to reduce the friction.
Unwounded String
The low E, A and D strings on your guitar are most likely wound, while your thinner G, B and high E strings are plain strings, i.e, unwound. Plain strings tend to go out of tune more easily.
Other General Reasons
String Tension
Your strings are all under tension, but some strings are under greater tension than others due to the note they are tuned to relative to the width of the string. The D string is under the most tension, but since it is wound, it copes with it well. After that comes the G string, but since this is a plain string, the tension can cause tuning issues.
Broken Tuning Machines
If your tuning machines do not increase the tuning steadily as you turn them, or worse still, slip, i.e., as you turn them, the tuning increases, then suddenly drops, and you have to tune up again, then they need to be replaced.
This is a simple job for most guitarists as long as you buy replacement tuning machines that fit your guitar perfectly. If not, there is a lot more work involved.
Improper Set-Up
Make sure that your guitar has been set up properly. If you don’t know how to do this yourself, take it to a professional Luthier. Not only will it stay in tune better, as well as be perfectly intonated (in tune on every part of the neck) afterward, but it will also sound better and be much more enjoyable to play.
Maybe your guitar hasn't got a proper setup in the first place. Poor intonation can affect much more than just an open string being out of tune, as chords and even single note lines can fall flat or sharp.
Old Strings
Old worn-out strings are nearly impossible to get in tune, and even when you do, they won’t stay in tune for long. So change them! Strings are very affordable and will make your guitar sound and play much nicer than it did with the old lifeless strings on it.
