The tune can be found here:
Main chord progression is:
Bm7 - D9 - Gmaj7 - F#7b9
My understanding is that it's actually:
i - V/vi - vi - V
With D9 being secondary dominant for Gmaj7.
However, something strange happens later, the harmony changes in the main part:
Bm7 - D9 - Gmaj7 - F#7b9
E11 - E9 - Gmaj7 - F#7b9
Are we still in the key of Bm7?
Where did E11 and E9 came from?
Let’s take a look at an E11 chord (or an alternate name, an E9sus4). What do we have? An E root then assuming the 3rd is omitted (good move and usually done) B-D-F#-A or a Bm7/E, a 1 chord with 4 in the bass in Bm. The next chord, an E9 basically gives you a 4-3 voice leading between the two. Now in Bm the diatonic 4 chord is E minor unless you use the ascending melodic minor scale, in which case it’s an...E7! The E7 IV chord is also a diatonic chord if you’re in the Dorian mode. What’s my point? They’re basically IV chords. Non-diatonic, yes but few songs are completely diatonic anyway.
MANY songs incorporate a IV7 in them. A few examples are “Mr. Magic”, “Oye Como Va”, Almost every blues tune ever written and pretty much every James Brown song that goes “to the bridge” (the IV7). The middle section of this song where it goes to E for a while can be interpreted as a modulation and that’s legitimate but to me it’s just hanging out on the IV chord for a while.
I know what you mean, but there's no 'key of Bm7'. There's a key of B minor, and that's indeed where we seem to be.
There's a simple and sufficient explanation that E11 and E9 are chromatic chords. That IS a thing!
Some would be happier if we justified them as being 'borrowed' from some other key. Sorry, I don't want to look at it that way. It isn't helpful. They're chromatic.
To my ear, the two E chords are a temporary, direct modulation to E major. They take advantage of the root movement from G to F# and also serve to set up the bridge. (Note that jazz and blues allow for the interpretation of a major key even when the minor seventh is present.[1])
In a different context, E might be interpreted as IV borrowed from B major, but while it is literally the case, I don't think it explains the overall sound any better than just thinking in terms of root movement. Similarly, other "standard" explanations, like treating the F#7 as a common-tone chord with E, I think also stretch the theory too far.
[1] Although Jazz, blues, and popular music harmonies are often in line with the common practice model of tonality, the don't adhere strictly to it. The presence of the ♭7 in major is a good example: it is frequently treated as a consonance rather than a dissonance. The bridge of this piece is a perfect example. The underlying harmony is E7, but the whole section is clearly perceived as E major rather than A major, as common practice tonality would demand.
