Altering Dominant Chords & Tritone Substitution

Before we start you should be aware of the function of your dominant chord. The dominant chord by itself is unstable & usually needs to resolve to a more stable chord a perfect fourth above, but that's not always the case as sometimes they play a static role where they are not intended to resolve to a specific chord. 

Here is an example to make it clearer:

Cm7 F7 Dm7b5 G7

The F7 is a static dominant as it doesn't function as leading chord, if it is so, it should resolve to some Bb chord, while the G7 chord is a dynamic one as it is intended to resolves to Cm7. Alteration & tritone substitution are applied only to dynamic dominant chords; that is when you find a 5-1 or a 2-5-1 progression (don't forget... the five is always a dominant chord) you are free to do some alteration.

As mentioned before the interval between the third & seventh of a dominant chord is a tritone (six semitones) & sounding these two notes simultaneously creates tension which is released by resolving to an appropriate chord. Dominant chords resolves to both major & minor chords a perfect fourth above, for example a G7 chord could resolve effectively to either a Cmaj7 or a Cm7. Notice that the tritone interval has disappeared in the last two chords, and that the bass movement from G to C is very strong, therefore this resolution is ideal.

Because of their leading function dominant chords are always placed in the last measure of the song to guide our ears to the home chord.

In Jazz you rarely find a dominant chord played in its basic form, extensions are always added to enrich the sound of the chord, these extensions are ninth, eleventh & thirteenth. You can alter the fifth as well as the extensions are altered, you can almost do any changes to the chord but you have to keep three notes unchanged: the root, third & seventh, all other notes could be altered freely according to your own taste.

It is clear that the second & the ninth are the same note "A" but one octave apart, it is so familiar in Jazz to invert chords & a Cmaj9 is not necessarily played "C E G B D", you may omit the root & play "D E G B”, there are countless possibilities for inversions & the chords by the end of a solo are sounded in a different way than at its beginning to make some sense of development, so to make it easier you can think of the 2nd & 9th, 4th & 11th and 6th & 13th as the same notes, just get the right notes &  play .

Now as you've got your extensions, it's time to add some colors to them. This is done by alteration. For example if you alter the 9th of G7 your options are:

G7b9, G7#9 & G7b9#9

You may alter the 5th also to get two new versions of each one of the previous chords; the diminished & the augmented, for the first chord this will be:

G7b5b9 & G7#5b9

The last chord works very well in a minor key.

The 11th extension is a tricky one & is rarely used, perhaps the 7th#11th is the common use of it outlining a Lydian dominant mode.

The 13th can only be lowered, if raised you will get a chord note (the 7th), in the key of G the 11th is an "E" if raised it's an "F" which is already a chord note.

The tritone substitution is simply replacing a dynamic dominant chord with another dominant chord a tritone above (or below.. it's the same!) this may sound strange but it works very well & here is how:

Taking G7 as an example, when substituted it is replaced by a Db7, let's have a look on the two chords construction.. 

The functional notes in a G7 (G B D F) are B & F, whereas in Db7 (Db F Ab B) they are F & B, the same two notes! Where the 3rd of the first chord is the 7th of the second one & the vice versa, therefore the function of a G7 could be accomplished with a Db7.

Dm7 Db7 Cmaj7

The chromatic bass resolution is very strong & leads our ears directly to the main chord.

Eb7 Dm7 Eb7 Dm7 Eb7 Dm7 Em7b5 A7 Dm7

Where the Eb7 is the tritone substitution of A7