[HANASHIR:1568] Re: tritone substitution

[Rglauber <Rglauber...>]

OK Emily, since you asked.....

The tritone substitution is one of the basic ways that jazz musicians (and
others) spice up a chord progression.  It has to do with the tritone,
otherwise known as the interval of a flatted fifth.  The reason it's called
the tritone is because it consists of three whole steps (tones).   If you
start on the note C, to find the tritone, you go up one whole step to D, one
whole step to E, and a third whole step to F#.  So the interval between C and
F# is a tritone (or flatted fifth).

Now there's something very special about that particular interval.  There is a
tension inherent in the tritone, and it's a tension that wants to "resolve" to
a more harmonious state.  In fact this tension--resolution is at the heart of
our western music, classical, rock and roll, jazz, country, blues, it all uses
the same basic building blocks, the major one of which involves the
tritone....

Let's say we're playing a song in the key of G.  Nine times out of ten, the
end of the song will go something like G D7 G.  That D7 chord resolves
extremely strongly to the G chord to give us a sense of completion and rest.
Everyone can sense it instinctively, everyone can hear it easily.  But if we
want to know WHY this is so, the answer is the tritone.  The D7 chord contains
the notes D  F#  A  C  (I  III  V  b7).  The key notes here are F# and C, in
that they create the "tension" that just needs to be resolved, in this case by
going from the D7 chord to the G chord.  (Yes a straight D chord will also
resolve comfortably to a G chord, but it doesn't have the compelling force
behind it that a D7 chord has)

Here's where I'll lose some of you  (assuming anyone is still reading, that
is), but the intent of this is to explain the "tritone substitution".   F# and
C exist in the D7 chord as the third and flat seventh of the chord.  They also
exist (in reverse order) as the third and flat seventh of the Ab7 chord.  (Ab
C  Eb  Gb(F#)).  It's still a tritone, and it still wants to resolve, still
needs to resolve.  

The "tritone substitution" allows you to substitute an Ab7 chord for a D7
chord.  So that G  D7  G could be altered by playing G  Ab7  G.  How do you
know that the "substitute chord is Ab7?  It's a tritone away from our original
chord of D7.  Here are some basic tritone substitutions, and if you understand
these, I'm sure that you can complete the list.
C7    F#7
D7    Ab7
E7   Bb7
F7   B7
G7  Db7 

Final thought.  These substitutions don't maks a lot of musical sense on their
own.  In other words you wouldn't play "Frere Jacques" and employ the tritone
substitution.  This type of harmony works best if you are already in the
process of jazzing up your music.  Also, this substitution is only a starting
point, it's not the end in itself.  There are many many ways of substituting
for a D7 chord, and the correct choice depends entirely on the musical
context.  The real use of tritone substitution occurs when one is aware of the
basic ii  V  I  way of jazzing up popular music.  If anyone is interested in
that idea, I'm happy to share.  

If anyone is upset by the length or subject matter of these "theory sessions",
I'd like to know that as well...

Rich Glauber