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I’ve always taken this for granted but maybe I’ve been doing it all wrong.
How are these thing suppose to correspond to one another?
String spacing at the saddle should be equal to ?????
12 fret width?

It has thrown me off because the string spacing at the bridge on my Martin is = to the fretboard width at the 15th. Strings are equidistant from the edge all the way down. Standard Martin scale. 12 fret width is 2.13”
The bridge pin spacing is

A standard (I thought ) fretboard I just made is considerably wider (2.25”) at 12.

1 3/4” nut.

I believe the rule of thumb is that string spacing should be the width of the fingerboard at the body join…

actually width at the 12 th fret determines the width at the bridge

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

If in doubt just get some paper and draw it. I use freezer paper taped to the bench. Each guitar gets new paper anyway so it’s convenient for drawing and scribbled measurements.


Steve

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Steve Smith
"Music is what feelings sound like"

Get thee to the Cumpiano/Natelson book. There is a method in there for drawing it all out on paper so that everything fits.

12th fret width equal to bridge spacing usually works out nicely. I like 2.25" for both. Gives a little more edge space as you move up the neck and the strings become more flexible and easier to fall off the edge.

If you make 14th fret width equal to bridge spacing, you get a more constant edge space.

I was taught bridge pin spacing should equal width at the 12th fret at the Charles Fox course back in 2003. That is always what I have done.

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It's not what you don't know that hurts you, it's what you do know that's wrong.

The fret at which the string spacing at the saddle will be equal to the width of the fretboard is a function of the nut width and the setback. Any rule of thumb for this will work well enough within certain setback and nut width values, but if you ever need to change those dimensions you may find that the rule of thumb doesn't quite work. The geometry here isn't very complicated, and I fully agree with Steve that drawing it out is a good course to take. The shape of the fretboard is a simple trapezoid. Extending the side lines down to the saddle location makes for a longer trapezoid. String setbacks on both sides can be represented as lines parallel to the fretboard side lines and at whatever offset from them that you or your customer like. Offset lines parallel to the setback lines at 1/2 the diameters of typical high and low E strings will locate the E string centerlines at the saddle.

The "G" Thang Acoustic Guitar Design Tool for Windows computers will generate a CAD drawing of all of this based on a few dimensional values. The tool can be downloaded for free from my website here:

https://liutaiomottola.com/formulae/gthang.htm

But as Steve said, it is really simple enough to draw full size on paper.

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R.M. Mottola
LiutaioMottola.com

Author of the books Building the Steel String Acoustic Guitar and Mottola's Cyclopedic Dictionary of Lutherie Terms.

I've only ever pulled these from a full sized drawing, so I can't suggest a formula. But, to me it has to come down to the fretboard taper and e to e measurement at the nut. Imagine no taper... the nut, 12th fret and saddle would be identical. Then the more you taper, the more exaggerated the difference between the nut and saddle spacing, with some pivot point in the middle... likely the 12th fret. I guess the point I'm making is, get the fretboard (neck) taper you want and lay it out in full size from there.