an explanation of horizontal tonal gravity?

[MokshaIs]

Would someone please present a brief synopsis or condensed understanding of horizontal tonal gravity? What I'm seeing so far is that any scale with the perfect 4th is a "horizontal scale" due to the gravitational instability of the 4th within that particular tonal center's gravitational pull... Is this a correct understanding?

[guitarjazz]

Yes but the way I think about HTG is just the key of the music. There are other factors but that one is easiest to grasp. Think of the melody to Stella. Even with all those chord changes the melody, with the exception of one note, stays within one major scale.

[chespernevins]

With Vertical Tonal Gravity, the chord of the moment is the gravity centering element.

Take the progression:

|D- G7 | C |

This translates to:

| F Lyd | C Lyd |

We would choose our melody based on each chord, so our melody
would come from F Lyd in the first measure and move to C Lyd in the 2nd measure.

===

With Horizontal Tonal Gravity, we still have chords (the harmony) based on the same 2 Lyd scales:

| F Lyd | C Lyd |

but we choose our melody in a different way.

To begin constructing our melody, imagine that the 2nd measure is simply a C triad.

| ...... | C triad |

The C triad is our gravity centering element.

Now choose the melody from scales that RESOLVE to that C triad (as opposed to scales that are CONSONANT with the C triad).

Scales that resolve to a C triad are going to be horizontal scales, and these include
C blues, C major, C mixolydian, C mixolydian b6, C maj Bebop, C Harmonic Major, C Aux Dim, etc.

Horizontal scales usually include the natural 4th degree of the scale, and/or the flat 6 of the scale.

So choose a melody from any of these scales, or a combination of these scales, or chromatically enhanced versions of these scales, and play it over:

| D- G7 | C |

You are now playing horizontally.

In some ways, playing horizontally demands more care than playing vertically. Horizontal scales must be played musically, so that they resolve convincingly to the final chord. Vertical melodies are more "foolproof", in the sense that any note from the scale can be held against the chord and sound consonant.

[guitarjazz]

Great post Chesper!
I've always thought that the melody to Goodbye Pork Pie Hat was a good example of Horizontal Tonal Gravity.