Arlington Fret Works

Perfect intonation for a guitar would mean that all the A notes on the fret board would be in tune with each other, all the B flat notes in tune with each other, the B notes, C, C#, D ... etc.   Perfect isn't possible on the guitar, but the closer the better.   

Basically each string needs to be adjusted at each end so their harmonics line up with the frets. So, for good intonation you need properly aligned saddles, a cleanly filed nut, and frets with crowns in the middle.  

Steel strings guitars need compensated saddles in order to play in tune. Each strings has a little stiff area at the contact point at each end, and the thicker and stiffer strings need more length to compensate for these dead ends.  This is why steel string acoustics have slanted saddles; the thicker strings are stiffer and need more room to move. Actually the plain B string is stiffer than the wound G string, so a compensated acoustic saddle has the B string contact point moved back. Nylon strings are very close to each other in stiffness, so classical guitars have straight saddles, and a compensated classical saddle has only a small adjustment built in.

Usually the G string on 6 string electric guitars is unwound, so it's stiffer than the D string. Then the saddle pattern is a  3-3  Z shape like this: 



The string slots in the nut need to be filed with a slope, front to back, so the strings break off the very front of the nut. Sometimes the slots get worn and the break point is farther back in the slot, away from the first fret. If you can wiggle the string side to side in the slot, it's worn. Then even if you adjust the saddle so the 12th fret harmonic is correct the 7th and 19th fret harmonics will be off, and the guitar won't play in tune.  This problem is pretty common.  Usually filing the slot is all that's needed, unless there's a chip or the slot is too low.

The string slots also need to be the right depth. High action at the nut means you basically have to bend a note to reach the first fret, and this throws off intonation, especially for the "cowboy" chords. You can tune the guitar so the G chord sounds good, or the D chord, but not both.                  One customer had a new Rickenbacker 12 string that wouldn't play in tune without a capo. Everything else about the set up was great, but the strings sat about .045 inch above the first fret. I lowered them and the problem was solved.             

Ideally the frets should be crowned, so the strings contact them in the center. If they're worn flat the contact points will be at the front of the flat area. It's like moving the frets away from the nut, throwing off the scale of the neck.  

This 1964 Gibson ES330 is a good example of a guitar with intonation problems. It had worn string slots in the nut, the saddles were adjusted wrong, and the first three frets had been worn flat.  The owner plays in different alternate tunings.  He's a very patient fellow, but was finally frustrated enough to have it looked at.

He uses a wound G string, so the saddle pattern should be 4-2, like on an acoustic. On the ES330 the patten looked like this:

The first thing I usually do is put a capo on the first fret and adjust the saddles so that the fretted notes match the harmonics at the 13th and 8th frets. This takes the nut out of the equation. Then I remove the capo and check the notes at the 12th and 7th frets. If these are in tune then I know the nut slots are ok. If a string plays sharp then its nut slot needs to be filed.  The dirt in the slots of this ES330 indicated this might be a problem, but the first fret was worn so flat I couldn't get an accurate reading with the capo on.

 
The first thing to do was to crown the flat frets, in this case the first three. 

These were so low that my crowning file wouldn't work, and I had use a triangle file.  

Now I could adjust the saddles with the capo on. After removing the capo I found that the A, D and B strings read sharp.  While filing them I could see that the front part of the slots were in fact lower than the middle portions. 

I like to file the slots at an angle about half way between the plane of the frets and the plane of the peg head. This assures good even contact across the nut and a clean take off point at the front edge.  I put a thin pencil line in the bottom of the slots as a visual reference while filing.  


The final, corrected, saddle locations. These are hand made bone saddles so the pattern isn't as clear as it would be with machined saddles, as the tops are different thicknesses. The contact points do follow the 4-2 Z pattern for a guitar with a wound G string.  

To stay in tune the strings need to be stretched thoroughly. I go up and down each string 2 or 3 times:

About strings, sometimes even a new set will have one bad string that doesn't vibrate properly. One customer with a Taylor acoustic  said the intonation on the A string had "gone off". The saddle was fine, no chips, but the problem had persisted through several string changes. It turned out all the strings were from the same 3 pack, and all 3 sets had defective A strings. A new string from my drawer worked fine, so she installed a different brand and the problem went away.
 

A final note. I'm a big fan of the Earvana compensated nut. The saddle needs to be compensated, so why not the nut.?  I found a dramatic improvement in intonation, like I'd never heard in any guitar before, and now I've installed them in my three favorite guitars.  Here's their page about theory: http://www.earvana.com/technology.htm