Drop-2 Chord Fun Facts

FF#1: Outer Limits

We’re going to talk a bit about seventh chord and four-part harmony here, so to keep things consistent, I’m going to refer to SATB for voicing, and keep pitch and string sets ordered from higher pitch to lower pitch.

Drop-2 seventh chords are formed by starting with a close position seventh chord and dropping the second note from the top (in SATB, the A) one octave. In the original close position chord, S and A are adjacent chord members. Now, however, S and B are adjacent chord members. So, we end up with SB pairs of {R,7}, {3,R}, {5,3}, {7,5}. In other words, between the outer voices of a drop-2 seventh chord, you always have some type of 10th or 9th.

Interesting. So, if that is going on between the outer voices, SB, what is going on between the inner voices, AT?

FF#2: Dyad Pairs

Now that we know what is always going on between the outer voices, what else can we say about how these chords are constructed? In the original close position voicing, adjacent voices, e.g., {T,B} are always adjacent chord memebers, e.g., {3,R}. But in drop-2, because of that octave displacement, adjacent voices are adjacent chord members + 1, e.g., {T,B} is now {5,R}. That means that {S,A} must be {3,7}. Well, with the knowledge of this FF and also FF#1 up above, these chords are now very easy to spell. In all four inversions, they are:
{R,5} {3,7}
{3,7} {5,R}
{5,R} {7,3}
{7,3} {R,5}

In drop-2 chords, root and fifth always go together, third and seventh always go together. This is worth memorizing.

FF#3: A Chain of Double Appogiaturas

Occasionally in jazz and popular music, we see harmonic motion by descending diatonic 5th, e.g., ii-V-I, or vi-ii-V-I, or I-IV-vii-iii-vi-ii-V-I. In four-part harmony with seventh chords, between adjacent chords we can say the following: two notes are common, the other two notes will descend a diatonic second. In fact, you can think of each of these chord pairs as double 4-3, 2-1 appogiaturas on the resolving chord. Think of the ii-V progression in Satin Doll for a good example of this.

To see how this plays out with drop-2 chords, let’s do the following. We’re going to play I-IV-vii-iii-vi-ii-V-I in F major, on the {2,3,4,5} string set, starting with the {3,7} {5,R} dyad pair. In the first progression, the common tones are between the outer voices SB, and the appogiatura happens on the inner voices AT. Next progression, the inner voices AT have the common tones and the outer voices SB have the appogiatura. From there, the cycle repeats.

FF#4: Those Dyads Again!

Let’s think about what just happened. We started with a I chord with the R member in the B voice. When we moved to the IV chord, the B voice became 5. The we moved to vii and B became R again. Wait, {R,5}, where have we seen this before?

What if we play the same sequence, I-IV-vii-iii-vi-ii-V-I, this time in Db major, same string set, but this time starting with {5,R} {7,3}. So we start with the I chord with 3 in the B voice, and next chord, the B voice becomes 7 of the IV chord. Wow, really? so now the B notes will alternate on the {3,7} dyad.

So, it looks like in a progression between any drop-2 seventh chords descending by diatonic 5th on the same string set, we have the following transformation R->5, 5->R, 7->3, 3->7. If you want to keep in simple, just remember that in the bass, {R,5} will alternate and {3,7} will alternate, depending on where you start.

FF Bonus Round:

Take any drop-2 seventh chord on the string set {1,2,3,4}. Drop the S on 1 two octaves, so that it becomes the B on 6, on the resulting string set {2,3,4,6}. What have we here?