I was wondering if there was either a name or a notation for a chord progression that consists of the same chord played one semitone lower each time.
Here are some examples to clarify my question:
In PiLOT it seems to be Am9 -> Abm9 -> Gm9 and in Chega de Saudade I think it's Bm7 -> Bbm7 -> Am7.
Could they be called passing chords? It seems like it is often used as a funny way to resolve a progression.
These are ultimately just passing chords, but one more specific way of understanding this is as a tritone substitution; see What is tritone substitution?. But this understanding uses an expanded view of tritone substitution; for that, see Must a tritone substitution use a dominant functioning seventh chord?
In short, this more liberal view of tritone substitution is just replacing a chord with a chord rooted a tritone away.
If we look at your two progressions, we could create a smooth circle-of-fifths progression by replacing the middle chord with a chord rooted a tritone away. Thus your A–A♭–G progression could be understood as A–D–G, just as your B–B♭–A could be understood as B–E-A.
By replacing those middle chords (A–D–G and B–E–A) with chords rooted a tritone away, we add some chromaticism to an otherwise completely diatonic circle-of-fifths progression. And by using this tritone substitution, we create a progression that just descends by half step. You can see this especially with a longer view of the circle of fifths:
B–E–A–D–G–C–F
If we replace every other chord with a chord a tritone away, we get
B–B♭–A–A♭–G–G♭–F
So far three possible labels have come up:
All the above may be appropriate depending on the harmonic context.
I think you want to consider the harmonic rhythm which is the rate at which proper chords change. In many, many songs the harmonic rhythm is usually one or two chords per bar. When there is motion happening much faster than the harmonic rhythm, it often makes sense to regard it as embellishment, some kind of passing or auxiliary motion, even if it incidentally forms "chords."
The other thing to consider is functional harmony, basically your flow of subdominant, dominant, tonic chord progressions. A lot of harmonic passages can be viewed as various elaborations of otherwise simple, functional progressions. A fairly common thing is to start on a tonic chord, move voices by little half step changes through a series of dense, chromatic seventh chord harmonies, which eventually arrive on a dominant chord to end the phrase. You can label all the chords, but don't overlook the importance of the phrase being and elaboration of I ... V.
A series of tritone substitutions can definitely create a series of parallel descending chords. If you're being picky, those chords should be some kind of dominant. But, as Richard points out in his answer, some jazz folks use tritone substitution to "explain" chromatic progressions, even when the chords are not functioning as dominants.
Ruby My Dear by Thelonious Monk provides a nice example combining functional harmony and a parallel passage - although it uses an ascending motion, not descending. The first two phrases are just ii V I sequenced up a whole step, but between those two phrases, while the melody holds a whole note, there is a lead-in of parallel minor seventh chords. That parallel passage is just embellishing the main chord structure, it's a type of passing motion.
In the examples you linked, if I'm hearing the right moments, they happen fast, and feel like they are between the main chords. That's passing motion.
Using adjacent chords of the same shape is called 'planing'.
Yes, I suppose they're a special type of 'passing chord'.
Yes, they COULD be interspersed with other chords (real or imagined) to make a string of I-V relationships.
But just 'planing' is often the only description necessary..
Here's another look at the idea:
It is the 'Wind Cries Mary' Effect. Used by Jimi Hendrix in the intro to the song 'The Wind Cries Mary'.
