Why does this chromatic walkdown bassline work?
The above song (in F) has a bassline that walks down by half-steps from F to C# and includes C, B, and A#. Why does it work? What's the theory behind it? I know that E and B make good blue notes, but what about D# and C#? Is it switching between major and minor scale?
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It works because harmony is the result of adding melodies, and a scale, diatonic or chromatic is a very strong melodic element. The opening of this tune, the bit with the descending chromatic scale, goes: F, C/E, F7, Bb/D, Bbm/Db, F/C. Very similar to the end of "When the Saints Go Marching In" which has become a jazz cliche. The only reason this might trouble your sense of theoretical propriety is if you had slipped into the error of thinking that anything beyond diatonic chords need special justification. I7 and iv (F7 and Bbm) are very commonly used, and need no excuse of "temporary modulation" or "borrowing". Other common chromatic chords in the key of F are Ab, Eb, D7, G7 etc. etc. I suppose the most "way-out" harmony in F major might be a B major chord. But even that gets used - functionally as a tritone substitution or the beginning of a "cycle of 5ths" journey back to F, or decoratively - sliding up to a C chord.
It seems the composer was limited to three voices, and his voice doubling is somewhat poor, so the lines here do not exactly outline full harmonies. There are probably a number of ways you could look at this.
That being said, it seems to me that we are hearing what is known as a sequence. Sequences are patterns that repeat at different ranges, often used to harmonize a melodic line (e.g. the descending 5-6 sequence in the Pachelbel Canon). The pattern in this piece is down a perfect fourth, up a minor third, and is used to harmonize a descending chromatic line by alternating between 53 and 63 inversions, making this a form of a descending chromatic 5-6 syncope, if you want to be technical.
In the key of F it would look like this:
F
C
Ebmaj7
Bb
Dbmaj7
F
G7
F
C
When chords follow sequential patterns, the "rules" that govern progressional harmony get a lax hand. Things that would otherwise sound strange end up sounding OK because everything else held together by melodic forces.
The sequence is followed by a modulation to the dominant. Normally pretty standard. In my opinion though that G major chord doesn't work so well, as it takes the pitch palette in a totally different direction (we've been doing flats so far, but now he throws in that sharp-- authentic vs. plagal motion on the circle of fifths) and isn't prepared in any way. Then he jumps into what seems to me a rather premature 64-53 half cadence. To me it just doesn't work, actually.
In F the notes are F, E, Eb, D, Db. They reflect a note from each harmony. F goes with F chord; E goes with C chord; Eb goes with F7 - (or poss. Cm7, or Eb); D goes with Bb chord and Db goes with Bbm chord. Whilst the D#/Eb and B are good blues notes, E isn't.
Out of interest, that 3rd chord would contain D# (not called Eb) if the chord was B7, used in a tritone substitution, - but here it's not.
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