Originally Posted by Impromptu
Drop 2 is traditionally used on a scalar line over relatively straightforward harmony, so you're thinking basically in terms of harmonising the home scale (whatever it may be at the time). Try experimenting with things like Frere Jacques or There Will Never Be Another You. Anything where the melody moves for a while in clear scale steps.
Point is, it helps to think of Drop 2 as a lateral process, rather than a vertical event. While any isolated chord can be "in" Drop 2, it's more useful to think in terms of "harmonising a line in" Drop 2.
While you can use the concept for any four-note chord voicing anywhere, the core principle is that you can harmonise every tone of the scale with inversions of either a tonic 6th chord (CEGA) or a dominant 7b9 chord (G B D F Ab).
You start with these chords voiced as close as possible. So, eg in C (melody on top):
C D E F G Ab A B
A B C D E F G Ab
G Ab A B C D E F
E F G Ab A B C D
Note 1: No root in the dominant chords, I hear you cry? Doesn't matter, the bass has got that. Much more important that the b9 is adding a diminished flavour to proceedings.
Note 2: There's your Andrews Sisters formula, btw - well, okay there were only three of em, but it's the gist.
Harmonically, you're hearing C6 G7b9 alternating with each other - oh and there's a half-step added between 5th and 6th to keep the tidy alternating pattern. This also leads you to the concept of the jazz major bebop scale - C D E F G G# A B. Although jazz didn't invent it.
(If you want to do the same in C minor, just alter the base tonic 6th chord from CEGA to CEbGA.)
So far, so ho-hum. But by spreading the voicing (literally dropping the 2nd voice down an octave) you get a richer interaction between tones - a much less obvious, traditional, organ-like "girl tied to railway tracks" sound.
Of course, once you've got the hang of the basic structure you can spend hours altering and messing with internal voice movements.
Other options (more commonly applied to band arranging than chording instruments) are "drop 3" and "drop 2 and 4".
Full big-band arrangements (standard being 5 saxes AATTB, 4 trumpets and 4 bones - one maybe a bass) routinely contain "drop" voicings in combination, both within and across instrumental sections. These ideas all ultimately derive from classical woodwind and brass orchestration, which likewise ultimately derives from the idea of interlocking choirs of SATB.
It's a wonderful subject this - explore and enjoy! You'll find so much detail and beauty in it. And as someone upthread mentioned, you'll never hear a bedroom merchant's hamfisted use of an "awesome 10,000 miffed orcs at midnight" orchestral sample quite the same way again!
Great to have so much info in one relatively compact place. I just saved this thread into a PDF for future reference!
Of course, I've often played many different inversions on both guitar and keys, but I've barely ever thought about what went into them at the time, you know, although I sometimes have gone back and analyzed particularly tasty bits to figure how they got that way. And while there was likely some intuitive grasp of the movement and relationships, I can't say that I ever consciously put much effort into figuring it all out. (That said, in the last few years, I've been using the DADGAD tuning on guitar a lot, and it seems to readily lend itself to enhanced knowledge/understanding of the harmonic relationships one is playing -- particularly since the tuning tends to push one to use a limited number of keys, particularly but not limited to D/Bm and F/Dm, and that can help if you're trying to keep track of the harmonic value of individual notes you're playing. [The latter being a discipline I avoided for decades.