First of all, I would like to say hi since this is the first question I've had on this website. I recently came across Officially missing you by Tamia and when I look up the chord progression on Ultimate Guitar, something interesting kept me wondering. The song is in the key of G and the verse goes something like G F#m-B7 Em7 Dm7-G7 CM7 Bm7-Em7 Am7 Dsus4 and the part confused me the most is the use of Dm7 in this progression, is this somekind of temporary modulation from the key of G to C (Dm7 to G7 to CM7 and then back at Em). And what kind of knowledge does this bit involve? As far as I've known, there are stuffs like modulation, toncization, modal interchange, modes, I'd be glad if you guys could point out what I should cover in order to fully grasp the idea and be able to create such interesting progression.
Tonic chord. Then go 'down one' and start a ii-V7-i7 towards E minor, 'down one' again then ii7-V7-i into C minor. Finally iii7-vi7-ii7-V7(sus), as classic a 'cycle of 5ths' back to the dominant of G as you're ever likely to see!
You can explain this by fragmenting it into a series of modulations to new temporary tonics. Or you can embrace it all within G major. Dm can indeed be ii7 in C minor. But it is also allowed to be v7 in G major.
If you've got a collection of 'hot licks' to play over ii-v-i it might be useful to look at it the first way. Or you can look at the broader picture and let each of those chords be what it is in G major.
'Notes outside the scale do not necessarily affect the tonality'. Walter Piston, Harmony.