If your guitar doesn’t play in tune read this article.
In the year 2012, factory manufactured guitars are made with incredible precision. For the most part, computerized machines make them. Clearly, there is still some handwork involved in the production process but fret slots, nut placement and bridge placement should be accurate. There will always be exceptions. Let us assume that those pieces of hardware are correctly installed on your instrument. Here is a list of adjustments you or a qualified repairperson need to check to insure that your instrument is properly intonated.
Is your nut carved correctly?
If have a locking nut go to the next item on the list. Locking nuts are made of hardened steel and rarely need to be replaced or carved. If your instrument has any other type of nut then read on.
You must check this adjustment first!
After approximately 6 months of playing most guitars, the string wears the base of the carved nut slot away from the face of the nut. The string is leaving its oscillation point further behind the nut face and increasing its scale length*. I have seen strings oscillate 1/16″ of an inch behind the face of a nut. It is very important to have your nut slots carved flush to the face. Sometimes your nut has to be replaced or shimmed if becomes to close to the first fret after carving. The bottom line is: A worn or incorrectly carved nut makes intonation difficult if not impossible. A good indication of a worn nut is when cords combining open strings and the first three fret are playing painfully out of tune. An example of this would be if a first position E chord plays in tune and an A or a D in the first position plays out of tune.
After the nut adjustment is complete you must inspect your frets.
Inspecting your frets is very important. Unless you own a Parker Fly you have to be educated about fret ware. Frets are usually made from a blend of steel and nickel. A guitar string is harder than the frets. Frets over time become dented. Little dips and crevasses will appear. The first signs of wear on most guitars appear on the first four frets. On basses it varies. If you are a funk player it will be on the upper frets where you slap. Then it will be where you finger your fret board most often. It all depends on the player and their technique. If the fret is not properly crowned the string does not leave the fret surface at the right point. Some frets on the fingerboard rarely exhibit wear. This is common, but you must balance the unworn frets with the dented ones. This is done by leveling or dressing the frets and then re-crowning them. A procedure done only by an experienced luthier* or repair person. If the frets are severely worn or you have a warped finger board that the truss rod can’t straighten then plaining and refretting the fingerboard is a must. You must have a straight neck that works with the truss rod in combination with properly honed frets to achieve accurate intonation. If you need to refret the fingerboard then you have a choice of picking a different style fret instead of the exact factory replacement.
Check the saddle or saddle blocks.
On acoustic guitars the saddle is made of bone or synthetic material. It must be carved so the string oscillates at a precise point. Exactly where that point is depends on the design of the bridge. Some manufacturers have the string leave the face of the saddle; others in the center. There are even compensated saddles designed to have every string oscillate at an exact intonated point. This can be custom made if accurate intonation is desired. It is not necessary however.
I have also seen many instruments with saddle slots carved incorrectly. It is very easy to fill the old slot and carve the correctly measured new slot into the bridge. If done by a skilled repair person it will be undetectable.
This is a part of the intonation process when servicing an acoustic guitar. So make sure you ask your repair person if they will check to see if the saddle is in the correct position and at the proper compensation angle.
The last thing you want to have looked at is the curve of the saddle in relationship to the radius of the fretboard. All of the above points are important for accurate intonation.
Electric guitars are more flexible in their adjustment. On most models you can individually adjust each saddle block for their height, length and over all balance to the fingerboard. Some saddle blocks are made of hardened steel and will never wear. Others need to be carved just like a nut. Most importantly, they have to be adjusted in order for your instrument to play in tune. Intonation rarely needs to be performed after it is done correctly the first time. Most electric guitar saddle blocks do not move back and forth from their positions on the bridge.
When do you need to adjust or check your intonation?
When you change brand, model or gauge of string. If you use a different string from your last intonation setting you need to intonate again.
Can you intonate your guitar yourself?
Yes of course you can. If all the correct adjustments listed above are made and you know your neck is straight. Then get your analog metered tuner and match your tuned 12th fret harmonic to the fretted note. If the fretted note reads to the right of center on the meter then your string is sharp and you need to move the saddle away from the neck. If the fretted note reads to the left of center on the meter then your string is flat and you need to move the saddle closer to the neck. If you play in a dropped tuning, intonate your guitar in that tuning. Not to standard tuning.
One more thing:
I almost forgot this one. Attention single coil players: Your pick-ups may be to close to the strings. Especially the 5th and 6th strings. If you have powerful or classic/vintage style single coil pickups they can effect intonation. Their magnetic field can pull the string sharp or flat. I always adjust the neck pick up lower than performing level when adjusting intonation.