Neck dovetail and truss rod barrel nut

[sunburst]

...20 different answers...
I’ve learned that quite a few highly-skilled people do exceptional things without being intellectually aware of how they do it... High skill, no ability to describe or understand what was involved...

There is a lot of that in luthery and it can lead to inefficiency. It drives me crazy sometimes! There is much that is dogmatic and much that is done because "that's the way it's always been done".
I've always been someone who wonders why and how, and with somewhat of a background in science (BS biology) I want to know why I do what I do. When I first went to work "at the factory" I would often ask "why do you do that?" or "why do you do it that way?" The answer would usually be "it works...". I'd ask "what causes this?" and the answer would be "I don't know...".
Eventually I started to figure those things out and found lots of inefficiency. A procedure to correct something that could have been done better in the first place, and inefficient way to do something that could easily be streamlined, that sort of thing.
If you ask those 20 or so builders why they do truss rods the way they do, if they're honest, I'm sure some will say "I don't know..." or "it works..." or "I've always done it this way..."

[Dick Hutchings]

Or, I learned it from the RS book :-)

[Eric Oliver]

Brings to mind the Monty Python movie segment in the armory, moving something like the rivet pan to a location more efficient for the worker and that sets off the catastrophic chain reaction.

[HoGo]

I wish someone would write a paragraph or two that explains the physics of truss rods. It seems to me that the string tension is trying to bow the neck upward and the truss rod is installed to counter that force. If that is true then l have some difficulty with a truss rod that starts near the heel, goes upward toward the finger board, then away from the finger board. Wouldn't putting tension on such tend to straighten the rod thereby also bowing the neck upward?

The easiest explanation that shows how the rod works is thinking of archery bow. The bowstring acts just like the truss rod in neck. If you shorten the string, the bow will bend more, but if you pull the string liek you would for shooting the neck will bow more, that is clear but if you push the string against the bow handle what happens? BINGO! just the same, the bow doesn't know which way the bowstring is displaced but the bow nocks feel that the bowstring loops want to go closer together and so the bow bends. The force needed for (small) displacement of the string is relatively small compared to the forces of loops against nocks at the ends of the bow so even bent rod works well (just like the bowstring displaced towards the handle). The part of force that wants to straighten the rod is negligible in this context and the position of the anchors (nuts) defines where the axial force is applied and the deeper from fingerboard it is the more effective it is.
You can also try an experiment of trying to stretch rope of some thickness and length (weight) to be perfectly straight. You will see that the weight of the rope will be enough to make it near damn impossible to stretch it enough. Actually to get it stretched into perfectly straight line would require force large enough to break it. (my gut tells me that in theory the force would converge to infinty as you approach straight line) This shows that if it is not too far from the straight the axial force is by far the dominant one.