Tell me if this makes sense. I've got a D2HA and an OM3. I've had the D2HA since about May of 2002 and the OM3 since April of 2003. For the past several months the hygrometers I keep in the cases (where I keep the guitars when I am not playing them) have been showing the humidity in the cases to be around 60%. Once the cooler weather started coming in a couple months ago and I've been running the heat the humidity in the cases has dropped to around 45%. Perfect actually. The guitars sound awesome. The OM has never sounded better to me. Now here's the thing I'm wondering makes sense. I have had to tighten the truss rod on the OM three times in the past month or so. It's been less than 1/8th turn each time. Last night I had to tighten the truss rod on the D because it felt and looked like it had a bit too much relief. It was definitely more relief than it has had since I've had it. Again, about 1/8th turn and now it feels perfect again. So, should the necks develop more relief as the guitars "dry" from the 60% humidity they've lived in the prior several months to the 45% they've been living in the last few? Does that make sense?

I use large humidifiers in my music room just to keep around 45%, so I don't know much about going from 60% down to 45%. I guess my first though would be the other direction, but I've never studied this enough to really know. BTW, what are those guitars doing in the cases?

Allen:

If you think about it, as the guitar gains moisture, the top swells. So the belly will swell a bit. You can lay a straight edge across the belly just at the lower side of the bridge perpendicular to the neck. Or a properly humidified guitar one should see space at the edges of the top. When the guitar swells, the neck will show more relief because the top is allowing the bridge to pull up a bit. Too much moisture will also throw off your neck angle a bit. When your guitar gets dry, the neck will show less relief, your neck angle will tilt back, and your strings might even buzz.

I may be telling your something you already know. If so I apologize. Taylor has the best explanations of this on their website under the tech sheets. Look under signs of a dry or wet guitar.

Answer to your question is that a dryer guitar will have less relief; a wetter guitar will have more relief.

It's wonderful to deal with wood, isn't it.

But this is completely opposite of what you describe, Jeff. The relief in the neck seems to have increased as the guitar "dried" from 60% to 45% relative humidity. Now you see why I was asking if it made sense that I would need to tighten the truss rod as the guitar dried a bit.

As I see it this really has nothing to do with the top moving with humidity. We aren't talking about a dry guitar in the classic sense and I doubt 60% humidity is high enough to cause any significant top disortion anyway. I think this is all in the neck.

If I hold the low E down at the first fret and the body fret I'm effectively taking the saddle height out of the equation. This is pure neck relief. If the top were settling some as the humidity went from 60% to 45% I would expect the action to become lower, but it "felt" like it got a smidge higher. When I check the relief as described above my perception was that there was more relief than there had been. I don't have any hard measurements as I've just learned to eyeball it on these two guitars.

I may be just imagining all this. Maybe I was tired and it seemed to take more effort to fret the strings. Without actual measurements it's hard to say if there was really more relief or not. It just seemed a little counter-intuitive to what we expect as a guitar dries a bit. Or does it? Might the neck pull forward a bit as it dries? Hmmm...

Allen:

Sounds strange to me. All I know is what I have told you from reading and what I see with my guitars as the humidity changes. More humidity means more belly and bridge up a bit, and resultant action. And when things get dry, bridge down, and action lower.

This is pretty hard and fast guitar geometry. Maybe something else is going on??

The bridge moving up and down with the humidity will change the action, but not the neck relief. The relief might also change, and might also change the action, but the top moving is usually more dramatic with moisture changes than neck relief moving. Especially in the stiff Collings necks.

With low humidity I can feel the ends of the frets on the side of the neck, so the neck has shrunk slightly (frets haven't expanded), and less humidity/smaller neck would create more relief in the fingerboard, and that's why truss rods are a wonderful invention.

So, yes, makes sense.

Allen:
I don't know about how long you waited to check things, but probably if the neck mahogany (more mass, different material) lost the extra humidity at a different rate than the ebony fingerboard it could cause a tiny bit of temporary torque for a short time.