What makes a "jazzy" harmony?

There is a specific sound you must achieve in order to make your composition be identified with a specific genre.

Today we will  talk about the sound in jazz.  How to achieve that specific sound?

Well, it all starts with the harmonic portion of the tune/song.  Jazz harmony is still based on tertian harmonic principles.  This is the same as to say that chords are built in thirds.  An example would be the chord of F major.  The simple triad is composed of F, A, and C.  These are the basic notes in you will need to make in any order to achieve the F major chord.  By the way, the F major (as with ANY major chord) is built by starting with the Root note, adding a major third (M3), and also adding a minor third (m3) consecutively.  Example:  F plus a M3 = A....now A plus a minor m3 = C.  You can get any major chord built using this formula!

Now, the jazz sound will be achieved by simply playing with the upper extensions of the chord.  As said before, the basic notes to get the triad are: F, A, and C (the 1, 3, and 5 in the scale of F major). By the way, chord formulas are expressed in relativity to the major scale or by the addition formula I mentioned above.

However, if we add notes to the chord such as:
the 9th (the same as the 2nd), the 11th (the same as the 4th), and the 13th (the same as the 6th), this will make the chord get a different harmonic color.  

In essence, the sound of harmony in jazz is achieved this way.  You must not omit the 3rd of the chord (A in F major), and keep the 7th as well.  Keeping the 3rd will let your ear know if the chord is major or minor, AND keeping the 7th will let your ear know if the the chord is dominant 7th or not.

The role that a dominant chord plays is important, so although you may be tempted to leave the 7th out, don't do it unless you want to achieve that sound artistically speaking.

Therefore, chord extensions are a big part of playing in that jazzy sound!