That's it!
from 9/15/2002, by ben blau
the a and b forms are specific voicings for ii-v7-i progressions, which are a cornerstone to most jazz compositions. they are rootless voicings that are particularly well adapted to the piano left hand, though they can be played by other instruments or even used in jazz orchestration.
the a form and b form contain the same chord tones for each chord, but are inversions of each other (so that they remain in the proper register in all keys). thus, "a" form voicings are generally used for keys c through f, and "b" form voicings are generally used in keys f#/gb through b. exceptions sometimes arise out of necessity, when voice-leading through a set of chords, but these exceptions make themselves evident without having to classify them.
here are examples of each form.
key of c major, a-form ii-v7-i:
(d bass) f a c e (3,5,7,9)
(g bass) f a b e (7,9,3,6)
(c bass) e g a d (3,5,6,9)
key of g major, b-form ii-v7-i
(a bass) g b c e (7,9,3,5)
(d bass) f# b c e (3,6,7,9)
(g bass) e a b d (6,9,3,5)
notice that in both forms, each chord voicing utilizes the exact same tones, with respect to the underlying roots. for example, the "ii" chord always uses the 3rd, 5th, 7th and 9th, even though they appear in a different order in the b form compared to the a form.
also notice that the voice movements themselves are the same in voice-leading through both forms. when moving from "ii" to "v7," the 7th of the ii chord always moves down a half step to the 3rd of the v7 chord. this is the only pitch that changes (except for the roots, of course, which you'll not be playing in this case). when moving from v7 to i, each tone moves down one diatonic scale degree. these are some memory keys that might help you get through learning them.
c form voicings are used for altered dominant chords. it's best to first learn about tritone substitutions to fully grasp their use, applications, and implications, but i'll do my best to describe them to you.
suppose you want to play a c-form altered dominant in a ii-v-i progression. instead of playing the a or b form voicing for the v chord, play the voicing for whatever dominant chord is exactly a tritone away. if in c major, for example, you'd play the voicing for db7 in place of the voicing for g7 with your left hand. notice that db7 is in a b-form key, and that the g7 is natively a-form. this will always happen when playing c form dominants -- they'll always be the "opposite" form from the key you begin in.
now, the reason these are not ordinary tritone substitutions has to do with the root employed under the voicing. if the bassist were to play a db root, it would be considered "subv7," or a tritone substitute of the original v7 chord. in the c-form, however, the root of the original dominant chord is preserved. the pitches contained in the b-form voicing for db7 over a g root yield an altered voicing for g7 -- specifically, g7#5(#9).
example:
db7 b-form = (db bass) f bb cb eb (3,6,7,9)
g7 c-form = (g root) f a# b d# (7,#9,3,#5)
notice how the tones of the c-form voicing for g7 are enharmonic to the tones of the db7 voicing.
i hope this gives you some insight into how the system works. if you learn these voicings, you'll find them a highly organized way to get your left hand prepared for a high percentage of the requirements of most jazz songs. that's not to say that there is no need for other methods or voicings, but this system provides an excellent backbone.
more information can be found in the book, "contemporary jazz styles," by john mehegan, as well as in the works of other authors, such as mark levine.
good luck in your endeavors,
ben blau
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