Basic setup questions

[tot_Ou_tard]

Let's say that I am checking the intonation by comparing the fretted third fret to the fretted fifteenth fret.

I know that if the the 15th is flat compared with the 3rd then moving the saddle towards the headstock will balance this out. If the 15th is sharp compared with the third then one should adjust the saddle away from the headstock.

My question is how do truss rod adjustments and string gauge affect things?

I don't know the correct terms for these things. But let's for the moment call a truss rod adjustment a "concave adjustment" if it gives more relief. That is, if it causes the neck to bend more like this



and call an adjustment a "convex adjustment' if it provides less relief.

So how do these affect intonation? Which one causes the fifteenth fret to become relatively sharper compared with the third?

Also how does string gauge affect intonation? Assuming that everything else is fixed (truss rod etc) does a higher gauge string tend to sharpen the 15th relative to the third or vice versa?



Sure, I could try all these changes & see what happens, but then I'd be taken to the looney bin where I'd spend the rest of my life muttering while making minute adjustments on the bedknobs of my hospital issued bed.


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Here are my guesses:

Higher string gauge will act like a concave adjustment, as the increased tension will tend toward providing more relief.

A concave adjustment will flatten the fifteenth relative to the third fret because increased relief will shorten the overall string length, but it will not shorten the distance between the 15th fret and the saddle by much. Thus, the relative distance between the 15th and the saddle and the third and the saddle will be greater which will be like moving the saddle away from the headstock.

Does your head hurt yet?

Thanks!