Action height discussion

[JCat]

I always thought that the 12th was used because it's convenient, and at the midpoint of the string. Geometry requires the string to rise higher above the frets the higher the fret number, unless the nut height is far higher than optimal. One could have the string be the same height all the way from the nut to the bridge saddle, but that would not be a pleasant instrument to play, and intonation would be terrible.

12th fret is the midpoint of the string, but not the midpoint of the neck. The actual bow of the neck (causing a tiny amount of relief) got its apex somewhere along the length of the neck, often in the spot where the neck meets the body (neck joint). But this depends on many other factors such as the leveling of the frets and the design of the guitar and neck joint.
It's possible to set up a guitar in fundamentally different ways, still reaching the same string height at 12th fret. This is why it's not a very useful measurement to describe "action".

[sgosnell]

Yes, guitars can be, and are, different. The truss rod's effect generally ends near the 12th fret, but not always. Any fret can be used for measurement, it just needs to be consistent for the guitar. That's why I don't think measurements are really necessary, and don't bother to try for any number, I just try to get the action as low as possible, and live with whatever numbers are the result. But I'm doing it for myself, not as an income producer.

[Rickco]

I think the basic answer is 1st it will depend on string guage and tension, more tension less buzzing with low action. 2nd is style, if your chunkin rhythm on an acoustic higher action will give you more volume with less buzzing. If you play electric volume isnt an issue and some styles will be better played with lower action and buzzing less noticable. Except for the above examples lower action is almost always more desirable. I will share this, as a kid in the '60s my friends dad played with Pizzerelli, Smith, Barnes, Cinderella, L Paul and others. We were young kids wanting to play rock but when I went over there once in a while him or one of the guys would let me pickup their guitar and strum it...yes JS picked out a guitar for him at the factory and I got to play it also the mid '40s D Angelico made for him by JD. and others, what was funny was they played the heaviest strings (we called them cables) and the action way up. This was the beginning of the light string age and we were even using banjo strings to make lighter top strings so it was amazing to see and hear these guys effortlessly glide over these instruments with the most amazing sound...when it was my turn the action was so high and hard I could barely play "secret agent man". I wish I had a do over