Jazz theory is the underlying technical ideas behind the playing of
music in the jazz style (in reality, it's more along the lines of the
bebop style). The fortunate (and unfortunate) part is that pure
technical players can't properly perform in the jazz style, as it's not
all theory. Jazz is also about self expression and understanding
that the theory isn't always right. But those who know nothing else but
that, and fail to understand the theory also can't properly perform in the
jazz style. It's a discipline that escapes the grasp of many (myself
included... it's a long road).
Jazz tunes move through chord progressions just like any other song,
be it a classical tune, rock or even a polka. What makes jazz
different is that the artists understand the chord structure of the tune
in a very deep and unique way. They know that the structure can be
altered at the whim of the individual and not have it compromise the
integrity of the tune, or screw up the other players. In fact, the
choices that the rhythm section makes can deeply complement the melodic
line (either improvised or scripted). A simple example is the following
II-V-I progression that is extremely common in all forms of music:
Dm7 G7 CMaj7
That's very common, and pretty
dull most of the time. True jazz players
will read that and will
alter it, usually without even thinking, into a
form that is more
interesting to the player and more
enticing to the
ear. The following change is a simple one:
Dm7b9 Db7#9 CMaj7
Where the
color of the first
chord (the II chord) is altered to give it
a higher level of
dissonance. That dissonance is part of the raw
material that one can use to
construct a tune, in both the
harmonic and
melodic sense. With that said, the dissonance is extremely light in that
chord and is really considered a
consonant chord by today's standards.
The next chord (the V chord) is extremely important and contains a lot of
dissonance to the target
key but is strangely related to it
as it is the
flatted
fifth substitution of the V chord. Since the V
chord is the closest moving interval to the I chord, its parallel chord
(from the idea of a "parallel universe" of chords stated by
Bob Mover) is
also just as close to the I chord as the V chord is. However, since it's
a
semitone from the
tonic, it is in the
half step key which is highly
related to the I chord and provides an exceptional amount of
movement to
the resolution key.
That's about the smallest taste I've ever seen about jazz theory.
With any luck someone will add to this node and flesh it out. It's
probably far too complex to happen any time soon.