He is playing it over a
F minor chord. The key of the song is
Ab. It looks like he just playing extentions of the
F minor chord. What is a better way to explain this?
He's playing a pentatonic scale pattern, harmonized in fourths, which is a common device in jazz. Open fourths give an ambiguous sense of harmony even though, in this case, he never actually leaves the home key.
Ab Major =
Ab Bb C Db Eb F G and an
F minor chord,
F Ab C, is the
vi chord, consisting entirely of notes within the
Ab Major scale.
The primary (upper) notes of the solo are
Eb F Ab Bb C Eb F Ab, which is an
F minor pentatonic (also
Ab major pentatonic) scale, starting on
Eb. By using the pentatonic scale, both
E natural (the leading tone in
F minor) and
G natural (the leading tone in
Ab major) are avoided, giving a sense of tonal ambiguity even though all of the pitches are within the key/scale. This is enhanced by starting on
Eb rather than
Harmonizing the scale at the fourth below gives
Bb C Eb F *Ab* Bb C Eb. Note that the pattern breaks at the
Ab. This keeps the harmony within the pentatonic scale and avoids the
G natural (
Ab leading tone). This use of fourths also reinforces the tonal ambiguity while staying within the key.
This is a basic example of "quartal harmony".
The pianist McCoy Tyner, among others, is particularly well known for using quartal chord voicings. A clear example can be heard in the opening vamp on "My Favorite Things" from the album Coltrane: Live at the Half Note.
He plays the following notes (disregard the rhythm values):
X: 1 K: C L: 1/4 [_B,_E] [CF] [_E_A] [F_B] [_Ac] | [_B_e] [cf] [_e_a] [f_b] |
It is an (almost) constant interval moving up over the notes of F minor pentatonic (F Ab Bb C Eb). However, in this context I wouldn't call it "extension", but rather a passage, transition.