I am practicing some blues bass lines and there is a chord progression of C7 | F7 | C7. The notes played during the F7 chord are (the bass plays one per beat): F A A♭ D, followed by the notes G B♭ C C during a C7 chord. What is the technical explanation of the A♭ and D notes played during the F7?
Whilst jdjazz's explanation is way more technically plausible - and correct - assuming the bass is playing roots on each chord (`a not unusual situation); it is based on the circle of fourths, can I offer another solution?
There is a set of notes used in blues that is known as the blues scale. In fact, there are two, a minor Blues, and a major Blues.
The notes that comprise F major Blues scale are: F, G, A♭, A♮, C and D. The bar in question contains four of those six notes. It's common to use the notes from either Blues scale in tunes - as bass lines or melodies- and base the notes used on the root of each particular bar.
This walking bass line arises by adding V chords and a tritone substitution. Here's how we get there:
- First add a v chord for C7:
| C7 | F7 | Gmin C7 |
- Then add V chord for Gmin:
| C7 | F7 D7 | Gmin C7 |
- Then add a v chord for the D7:
| C7 | F7 Aø D7 | Gmin C7 |
- Then add a tritone substitution for the D7 chord:
| C7 | F7 Aø Ab7 D7 | Gmin C7 |
This produces exactly the bass line you've identified. Moreover, the tritone leap from A♭ to D sounds great and catches the ear.
This raises an interesting question: is the bassist thinking about all of this harmony when he/she plays this lick? Almost certainly not. If the bassist plays jazz, then he/she has probably played this exact lick thousands of times. The chord changes
| Aø D7 | Gm D7 | and using tritone substitutions were both extremely common in bebop--and much of the jazz that followed. In fact, I bet you can find this same bassist playing that exact lick over many different blues recordings. The lick was probably the result of (essentially) muscle memory, thanks to the bassist's familiarity with more complex versions of the blues form that pervade jazz.
(As an aside, the F7 and Aø7 are interchangeable, and legend has it that the Aø7 chord arose in jazz out of a trend where bassists reached the progression F7-D7-Gm-C7 and played the third, an A, over the F7 chord.)
TL;DR: the bass line implies more chord changes than just the F7 and C7, but whoever wrote the material you're reading forgot to explain it.
The bass line F A Ab D can be used for many different things, but the one thing that does not work is playing a simple F7 chord over the whole bar. It just doesn't work. The bass line disagrees with a plain F7.
Are you familiar with chord substitution tricks? On the written "C7" the bass wants you to imagine a substitution trick, preceding a C7 with a Gm or Bb chord. Try it on piano or guitar in any song where you see a C7 chord: play a Gm first and then C7.
This is what the bass line sounds like to me. First there's just a bass, and then I play out the chords the bass line brings to my mind.
Here's another thing the bass line could sound like, if we don't want it to stay so bluesy. (opinions may vary on doing a Gdim7 in a too A7-ish way on top of the C bass note)
In my opinion the Ab note is clearly an indication of either an Ab7 chord, which is a tritone substitute for a D dominant (going to a G chord), OR it could be used for an Abdim7 which works as any of the four dominants: G7, Bb7, Db7, E7.
The Bb note is toying around with the "is this minor or major" thing that's common in blues. You mix things from C major and C minor, and keep the chords as dominant sevenths. I don't necessarily think "G minor" or "Bb major" when the Bb note comes, it feels more like "bluesish major/minor mix V chord going into a C". But it could just as well be a G minor chord to someone. That's the beauty of blues and jazz - it's ambiguous and the soloist gets to paint her own picture, using whatever freedom is left by all the other sounding things.
If you wonder why and how all this is done over plain F7 and C7 chord symbols - I take the "F7" and "C7" as just meaning "you know the good old blues thing that swings to this side and that side". It doesn't mean that you should play exactly an F7 chord or a C7 chord in a classical sense. They're just showing the stereotypical main stages of a blues.
My interpretation might be essentially the same as jdjazz's interesting way to construct a harmony progression step by step, I just don't think about it in quite as small lego bricks. Tim's blues-scale explanation might be just as valid - each note in the blues scale has harmonic properties baked into it. What you cannot do with scales though is select notes entirely randomly, because the ordering, placement and weight of notes in relation to the beat is very important in creating the sense of "are we more like at home, or leaning to the subdominant or dominant side".
I'm offering up a passing motion/non-chord tone type explanation.
C7 | F7 | C7/G
/G is my addition to make clear the quoted bass line in fact plays a second inversion chord.
(...nothing quoted for first chord) | F A A♭ D | G B♭ C C
The first two tones need no special explanation. They simply arpeggiate the
A flat could be heard as a chromatic passing tone to the
G. That also seems very straight forward and needs no reference to a scale or hypothetical chord. It's just the chromatic tone between two chord tones
D probably can't be explained satisfactorily as a non-chord tone in the bass. Preceded by a
C in could be a straight forward non-chord tone, but that doesn't happen in the quote. But, the
D is on a weak beat which could lend it a sort of 'not of the chord' quality. This single tone is the only one that seems to need a special explanation.
Not knowing the sources of the musical quotes, I do wonder if these chord symbols make more sense...
C7 | F7 | Gm C7
...which in part is how @jdjazz explains it.
Gm chord is heard, then the
D could be a sort of anticipation.
Occam's Razor take 5: Bb7 substituting for F7, or added to it. C7 | F7 Bb7 | C7...and then on to the rest of the blues...