Understanding Guitar Setup

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The Clothesline Effect

Many of you may have noticed more than expected resistance from the strings when fretting near the nut, especially at the first fret.  I would like to use the clothesline as a somewhat euphemistic analogy of this intonation issue.  I'm sure that you'll will prefer this to some trigonometric formulas.

Anyone who has ever hung clothes on a line has noticed that when you hang something near the middle of the line, the line sags a lot, yet when you hang something near the end, the line holds firm and resistant.

Stating this principle just a tiny bit more formally: The force needed to move a tight string a certain distance, perpendicular to the string, decreases as the applied force moves closer to the center of the string, and increases as the applied force moves towards either end of a string.

Now, the amount of force needed on a guitar string is the result of the the additional length the string that has to be stretched in order to gain the required perpendicular movement.  That stretch is maximized as you approach the ends of the strings, and that added tension affects guitar intonation.

There are several ways it relates to guitar intonation.  Starting at the nut, and moving towards the saddle:

If the first fret action is low, there doesn't seem to be any problem with intonation from this affect, because the movement of the string to the fret top is so slight.  With high action, however, there can be a measureable effect, causing the first few frets, particularly the first fret, to play sharp compared to the intermediate frets, regardless of other intonation efforts, other than displacing frets.  (This is distinct from sharping at the low frets due to lack of nut compensation).

As we move up the fretboard, toward the 12th fret, most of us are used to thinking that the amount of effort (and hence the amount of string stretch) is strictly proportional to the action height above each fret.  The clothesline effect tells us that that this is not the case - that the amount of stretch is considerably less than previously expected, as we move up toward the 12th fret.  So, in the mid range of the fretboard, we have to pull the string farther, but not harder.

This works out well for intonation up to the 12th fret, but beyond the 12th, the clothesline effect works against our intonation models, causing sharping at the high frets.  This sharping is very slight until about the 17th fret (we're talking electric guitar and top acoustic lead players here).  

I recently set up a Peavey electric with 23 frets, saddle intoned comparing fret 14 to fret 2, and compensated nut.  Except for one area on the low E, I was a able to achieve intonation within 2 cents all over the fretboard up through the 17th.  From the 18th to the 23rd, the pitch rose up to 4 or 5 cents sharp.

For a person who plays all the way up, we can intonate the saddle, at a little higher range, comparing, say, fret 17 to fret 5 or even fret 19 to fret 7 - I wouldn't advocate intonating any higher than that, because it would cause flatting at the mid frets.  For any sharpness remaining at the high end, It seems best to me to just accept it as a bit of stretch tuning!.  A little of sharpness on a lead gives it more "presence" - that may be a good thing (but I'm not the best judge of that).

Acoustic guitarists normally don't play that high, but acoustic guitar fretboards often recede away from the strings after the body joint at the 12th or 14th, aggravating the problem, and producing a similar sharping.  I would suggest that luthiers be mindful of that when working on fretboards of finer quality acoustic guitars, and on electric guitars.

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