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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?

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    Two different things to ask: (1) which underlying phenomena does this bass line utilize, and (2) what was the person thinking about who wrote this. The person who wrote it may just as well have taken it mechanically from a previously learned bassline or from a book. Even if the person was thinking about a scale does not mean that the scale is an underlying phenomenon in any other sense than "you can apply this note sequence if you see these chord symbols". – piiperi Jul 15 at 19:12
  • @piiperi - I guess that's pretty well what prompted OP to ask the question. – Tim Jul 16 at 6:05
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    I think the only way we can surmise what the composer was trying is to know what bar 4 (and maybe bar 5) are? In a classic blues he would be headed (back) to F7 in bar 5. How did he get there? – Bruce Kamolnick Jul 18 at 1:15
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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.

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    This makes me wonder if the bass line would still sound good after changing the order of the notes. I guess what I'm wondering if whether there's something additional (beyond just the scale and the notes) that tells us why the ordering of notes is important. I think that's where the harmonic considerations come in, but I'd be interested in your thoughts on this. – jdjazz Jul 16 at 12:08
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    @jdjazz - F D A Ab then to G works for me. Sort of tts on the Ab! There are so many combinations for walking basslines - my general rule is make beat one count then there's a BIG choice to get to the next beat one... – Tim Jul 16 at 12:19
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This walking bass line arises by adding V chords and a tritone substitution. Here's how we get there:

  1. First add a v chord for C7:

       | C7 | F7 | Gmin C7 |

  1. Then add V chord for Gmin:

       | C7 | F7 D7 | Gmin C7 |

  1. Then add a v chord for the D7:

       | C7 | F7 Aø D7 | Gmin C7 |

  1. 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.)

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    That's an interesting construction, but... seriously? Two secondary dominants hidden so well, one of them minor, plus a tritone substitution, in a blues tune? If this were jazz, sure, but in this case I highly doubt that the particular choice of notes had any such intention behind it, or that it would have a much different effect if the bassist had set up the chromaticism in another direction. — Then again, I don't know the song and you may be right here, but Tim's explanation seems much less far-fetched. – leftaroundabout Jul 15 at 9:45
  • (Thinking of it, yeah, there is of course the kind of blues that goes down the - circle even further than three steps, but they're generally putting them full-band in your face and only go to a minor to actually stay there a bit.) – leftaroundabout Jul 15 at 9:53
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    @leftaroundabout - ah, but where does Blues stop and jazz begin? This may well be a jazz-orientated answer, but that in itself doesn't exclude Blues use. There are indeed many jazz pieces which get re-interpreted using multiple V-I type changes - it's partly what the harmonisation of those pieces is about. Granted Blues is more often than not a 3-chord trick, but the OP asked for an explanation, and sure enough, got one! And it works theoretically (and in practice!). – Tim Jul 15 at 10:51
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    @MichaelCurtis, not necessarily. In jazz it's very common to hear both the original V7 chord and its tritone sub played side-by-side. – jdjazz Jul 15 at 17:16
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    @leftaroundabout, also a progression like this (which adds V's) became popular in bebop, which was played by smaller groups and which moves from one chord to the next extremely quickly. For good examples, check out virtually any Charlie Parker blues. Bass players aren't thinking about all the theory when they play; they acquire favorite lines and develop a vocabulary. That's how this sort of thing comes out in a setting where the song is more simplified. – jdjazz Jul 15 at 17:23
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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".

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I'm offering up a passing motion/non-chord tone type explanation.

Chords:

C7 | F7 | C7/G

...the /G is my addition to make clear the quoted bass line in fact plays a second inversion chord.

Bass:

(...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 F7 chord.

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 A and G.

The 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.

If a Gm chord is heard, then the D could be a sort of anticipation.

  • Then I shouldn't have deleted my comment after seeing jdjazz's answer after all :-) I thought I was being stupid suggesting that the Ab and D were passing notes to G and C. – Your Uncle Bob Jul 15 at 18:56
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    At the risk of bringing back 'Occam's razor' from the other comments it seems simpler to have one somewhat ambiguous D tone in the bass than to introduce 3 or 4 hypothetical chords. – Michael Curtis Jul 15 at 19:19
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    @MichaelCurtis, I think Occam's razor favors my answer. We find exactly this progression (| F7 | Aø D7 | Gm C7 |) in virtually every bebop blues. Adding tritone subs is one of the most common thing we'll hear bebop bassists do when walking those blues. The simplest explanation is that the bassist is familiar with jazz/bebop. I don't mean to suggest that the bassist is thinking about all of these chords--just that he's familiar with jazz/bebop blues and an extremely common line from that context slipped into this song. (For all we know, this is a Parker blues.) – jdjazz Jul 16 at 12:11
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    @YourUncleBob, I wondered about this too, and I think it's a totally valid thing to consider. We might test the idea that Ab and D are passing notes to G by asking whether it sounds as good to play | F A Gb C | G . (The idea is that, instead of approach G from a half step above + tritone, approach it from a half step below + tritone.) I like that lick but to me it sounds more harmonically stagnant than | F A Ab D | G . – jdjazz Jul 16 at 12:40
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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...

  • I like this idea, but it seems unlikely to me that the bassist would add in a new chord and not play the root. – jdjazz Jul 17 at 0:21
  • Fair point, but the 3 and the 7 of the chord identify it perfectly, and the line actually works nicely to bring you back to the C7 tonic, which is where you want to be in bar 3 of a classic blues. Tadd Dameron's Ladybird uses this, roughly. Close anyway. – Bruce Kamolnick Jul 18 at 1:12

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