When to use 6/9 chords instead of seventh chords

Question

I'm learning jazz piano from the internet, and recently I learned comping the song 'But Not For Me' with a walking bass in the left hand and the harmony in the right hand. In the tutorial, my teacher replaced all seventh major and dominant chords by the corresponding 6/9 chords, as well as all minor chords by 9th chords. This sounds very jazzy.

I then tried the same thing with other tunes such as 'A Foggy Day' and found that the seventh chords often sound better than the 6/9 chords.

Is there a general rule for how and when use 6/9 or 9 chords effectively?

The tutorial I mentioned is from Aimée Nolte's series called 'Accompany Yourself'. She is playing the song here:

But Not For Me

Note that the written chords are seventh chords, but she plays 6/9's or 9's and some altered chords, exactly as she explains here:

how to play major, minor and dominant chords when comping

Needless to say, Aimée is a wonderful teacher and what she's saying here works perfect (for a beginner like me). My question was merely: For what type of tunes does this method work best?

Answer 1

Without seeing exactly was done in the tutorial I will give you the following information. 6/9 chords are often used to substitute for major 7th chords (R 3 5 7). They are not called seventh major chords BTW, the word “major” comes first. Sometimes jazz fake books will indiscriminately use the maj7 symbol for a major chord but it is not always the best choice. For example, if the melody has a root note the 6/9 is a better choice because it avoids a possible m9 interval between the melody and harmony.

6/9 chords are NOT a good substitution for dominant 7th chords (R 3 5 b7) because they do not have the tritone of the 3rd and b7th characteristic of the 7th chord and will change the character of the chord progression. However you can add a 9 and/or a 13 (6) to a 7th chord depending on the melody.

Substituting a m9 chord for a regular m7 is commonly done by jazz players but there is one pitfall. Avoid the m9 on iii chords when the melody is the 5th degree of the tonic scale. It can create a m9 interval between the melody and harmony. Here is an example:

Avoid Em9 (iii) in the key of C when there is a G melody note. The F# can create unwanted dissonance.

EDIT: I’d like to add that the m9 substitution should be avoided on any m7 chord that has the third of the chord as a strong melody note (i.e. Dm7 with an F melody note).