..so got a question about action. I notice the action is a bit more consistent over the course of the neck on some of my guitars. Measuring the action on one of my Reverends I find 5/64's on the low E, 12th fret, and it's almost 6/64's at the 17th or so. Not much of a "rise" On my 6120 it rises a bit more. 5/64 at the 12th, around 7/64 at the 17th.
I'm assuming that is due to neck angle. Am I right, or should I be barking up another tree??
Just curious.....
Nope... you've got the right tree.
Well there's a pretty fine interplay between truss rod tightness, bridge height, string gauge, and even the angle the strings come off the bridge at ('break angle' I believe it's called?).
Getting the neck as flat as possible (less relief) while not fretting out is an art, and that gives you (more) consistent action.
Another big factor is the nut, how high and how well it's cut.
It has nothing to do with neck angle.
This has to do with neck relief and height of your bridge. I am guessing that the neck on the guitar with the more consistent action is less straight (so has some relief) than the one where the action becomes more over the length of the neck. If your neck is dead straight the strings will have to rise away from the neck because they start low at the nut. Otherwise you would have no action.
The neck relief (slight bend) causes the strings to able to be a bit more the same height over the whole length of the neck because the neck bends back to the strings if I may say it in a weird way. 
I disagree somewhat with Danman. Relief plays a more prevalent role below the twelfth fret. Neck angle comes into play as well and as is common on a lot of hollow bodies with elevated necks, there is a tendency for the neck to back bow slightly in the upper registers where there is no support.
Dan, you basically said what I said, but you don't think that's the neck angle? The angle of the neck relative to the body, I thought that was neck angle?
Hi Bigal:
I agree that we are probably saying the same thing.
But I don't agree on the neck angle part. The neck angle is only the angle at which the neck is put into the body. The height of the bridge takes care of that.
More neck angle > higher bridge.
Less neck angle > lower bridge.
But you are right that you have to also take into account that some fingerboards angle differently above the neck joint. I wasn't really thinking about that. And relief is more prevalent below the twelfth fret. But I saw it as just a place to measure it.
The fretboard above the neck joint can do all kinds of weird things. It can slant up or down.
Ok then we agree 100%, I just had my terminology wrong. 
He would have to measure the string height at several places along the fingerboard for us to know what the neck is doing and what the reason for the different set up is.
But is still think the relief is part of it. If the neck is dead straight the action will slowly rise all along the neck and thus also be visible up there. And if the neck has relief the action will sort of be the same higher up the fingerboard. Because the relief is the most in the area between the nut and the (sort of) 8th fret or so. Above that the string will almost follow the fretboard at the same height. That is if we don't count a fingerboard that does weird things above the joint.
These long typing sessions get the posts to overlap. Confusing sometimes. 
If you cut the nut slots down to the bare minimum, the bridge will have to be a bit higher to play well, and this makes for more of a rise to the action. I've found if you dont cut the nut slots as deep, you can have much more even-ness to the rise in action. If that makes any sense......You should ask yourself, where am I playing the guitar the most, first position? Way up the neck lead work? etc. Have it adjusted to where it feels best the majority of the time. Note, this is a deep subject and I have left out many aspects of what I'm saying, just to put a thought out there.
There is a wealth of knowledge in this forum that is just amazing as compared to any other forums. I learn something new here or expand my guitar knowledge every day. You guys are terrific... THANK YOU!
I'll have to measure the amount of relief on the neck of my 6120. The zero nut should take a too low nut out of the equation. Like I said, it's not a bit deal, my Gretsch is extremely playable, and it's not like I play much beyond the 15th fret. The action is great where it counts from the 1st to 15th frets.
Hi JBJ:
Basically that is true but wouldn't the nut slots have to be pretty high to notice the difference? If you make them too high the notes will be sharp when played at the lower frets and it would become harder to play at the lower frets. But you are right that if you get it right then you could help the action some there too.
Hi sroux,
Maybe you could even see the difference in relief between the two guitars if you look along the neck from the headstock down.
It's easy to see how much relief there is in the neck, especially with a zero fret. On a "normal" guitar, you capo the first fret and then press down anywhere from the 14th to the 17th fret. The distance between the string and the frets at the 7th to 9th fret is the relief. With a zero-fret guitar, the procedure is the same, but without the capo.
Too much relief can lead to excessively low action starting anywhere above the 10th fret, or excessively high action starting anywhere below the 14th fret. Too little relief can lead to buzzing low on the neck, particularly below the 7th fret or so. The less relief, the more accurate the intonation. String weight has lot to do with how much relief is desirable. Bigger strings, up to a point, vibrate more and have more of a tendency to buzz against the frets, but REALLY big strings are tighter and don't seem to buzz as much.
The best intonation comes from large strings, low action and as little relief as possible, even to the point of having none at all. The best sound (intonation considerations aside) comes from large strings and higher action. More relief can eliminate buzzing, but a guitar will be louder and more resonant with less relief, since relief lessens neck tension and decreases the transfer of vibration from the neck to the body.
Overall, I'd say it's best to keep relief to an absolute minimum and solve buzzing issues by raising the action or dressing the frets. On the other hand, if everything else is fine but notes and chords on the base position or slightly above it sound a little sitar-ish, it's probably best to introduce some relief and compensate by lowering the bridge slightly.
