Jethro Tull's "We Used to Know" is a great bluesy tune in 6/8 time, with the chord progression
Em B D A C G F#m7 B
These are quick changes, one chord per measure, so it's not like you can settle into anything.
What are some ideas for playing over this? On the album, Martin Barre lays down some serious wah-heavy lines, two separate solos, it's great.
First impression is that the tune jumps around a lot - it's not a diatonic progression, so you can't use one scale for the whole thing. But there's a pattern:
Em-B (i-V in Em) *Edit - fixed a typo
D-A (I-V in D)
C-G (I-V in C)
F#m7-B (v-I in B)
That last one is a little tricky; I'll come back to it.
Next thing I notice is that the modulations between keys of the first three are by minor thirds: B moves up a minor third to the D chord, A moves a minor third to C. Minor thirds happen to be the relationship between major and relative minor keys, so make a note of that.
Now let's take that basic info and start digging in deeper. Take the first couple of measures - they seem to be in E minor. Your chord tones here are E-G-B, B-D#-F#; if you line those up you get E-F#-G-B-D#... 1-2-b3-5-7, which matches up nicely to the ascending E melodic minor, the E harmonic minor, or the E Dorian scale.
Then you've got D-F#-A, A-C#-E. That's D-E-F#-A-C#... 1-2-3-5-7. You might have noticed there's no C in the chords of the first two measures - so it can be either C or C#. If we say it's C#, we can combine all four of the first measures to get E-F#-G-A-B-C#-D# - that's the E ascending melodic minor (sometimes called the 'jazz minor' scale).
Next you've got C-E-G, G-B-D. Since everything so far is moving by minor thirds, that major/minor relationship comes into play... the relative minor of Em is G. And there are two common types of cadences in music, the authentic cadence (V-I) and the plagal cadence (IV-I); we can see this chord change as either a I-V in C or a IV-I in G. Line up the chord tones here and you have G-B-C-D-E, or 1-3-4-5-6 - that's close to the G pentatonic scale, lacking the A note and inserting the C instead. Several scales will work over it, but E pentatonic minor (the same notes as G pentatonic major) seems a really safe bet.
Now we come to the crux of things...F#m7 to B. That's F#-A-C#-E, B-D#-F#. Assuming this leads back to Em, you're really looking at the ii-V part of a ii-V-I progression. So line those notes up around E: E-F#-A-B-C#-D# - that's 1-2-4-5-6-7. With the third missing, you can be back in E melodic minor. It's certainly going to have that minor character - G# hasn't appeared in any chord, and I'm guessing the pattern repeats, making E the clear tonal center.
So there's a structure that ties it all together, but you need to have a real feel for the progression - you're in E melodic minor for measures 1-4 and 7-8, but you want to lower the 6th and 7th in measures 5-6. Either use the natural minor here (which lowers both notes) or the pentatonic minor (which lowers the 7th and avoids the 6th).
That's one approach, anyway.
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