@AlbanyYEM said:

One question I have to any of the people with actual credentials on here is: don't all modes (Mixolydian, Ionian, Dorian, whatever) have a starting point dictated by the key? 10th fret for D, 5th for A, etc?

It depends on what kind of pitch you're talking about. You are correct that the 10th fret of the low E string is D, and the 5th fret is A. But remember that, on a guitar but also on any instrument, there are many "Ds" and many "As". Theoretically, there are an infinite number of them, although a finite number that we can actually hear. If we are talking about pitch-class, then you cannot say definitively that D is higher than A, you can merely say how far apart they are. But if we are talking about pitch, then yes, one is higher than the other.

@stufunk is right, D is both a perfect fourth above and a perfect fifth below A. However, in the case of Tweezer Reprise and Tweezer, the D of Tweeprise is, in fact, a higher pitch than the A of Tweezer. Listen to the album versions back to back and you'll hear this, Tweezer Reprise is higher.

Now, this would require some serious long term structural listening capabilities on the parts of fans, that we might have the ability to "hear" Tweeprise during an encore or set closer as actually higher than Tweezer that was played hours ago (or in the case of the recent SF shows or Hampton '09, days ago!). In reality, I don't think most phans are listening in that way (this has been debated endlessly by theorists and musicologists, whether we can actually recognize large-scale tonal shifts).

So what makes Tweeprise have the tension it does? Part of it is this basic fact: Trey plays the same riff over and over, while Page (in his initial run and the chords that follow) and Mike ascend over the first four steps of a D scale, playing major chords the whole time. The tension is created by the juxtaposition of Trey's stasis over the ascending chords from Page and Mike, and especially because those chords never reach the fifth degree of the scale, which would provide some measure of resolution because our Western harmony is based on fifth relations. When Trey starts ascending too, at the end, he is not doing so chromatically (as @AlbanyYEM queried) but along the major scale. He keeps pushing that higher and higher, and again, never reaching that fifth scale degree, which would provide some amount of resolution.

That's how I think it works. I definitely could be wrong, and would welcome any criticisms.

-Jake, Ph.D. candidate in musicology, CUNY Grad Center