Chord is a combination of 3 or more notes sounding simultaneously. There are many different chords: triads (5th chords) forming on 1st 3rd and 5th degree of the scale. Then there are chords made of 4 or more notes and they may contain 6th chords, 7th, 9th, 11th, 13th degree.
Triads (5th chord) – built on the tonic (1st degree) also called Root C-E-G, Subdominant F-A-C (IV degree), or D-F-A (II degree) and Dominant G-B-D (V degree). Those basic or “pure” sounding chords, without additional notes, are very rare in jazz music. You can find it in classical or folk music. In jazz, Instead of simple C major we usualy play C6, C6/9 or Cmaj7. For example C6/9 from the root: (C)-E-A-D-G-C, quartal chord.
Seventh (7th) chords – consists of a triad plus a note forming an interval of a seventh above the chord’s root. Chords that consists of major 3rd and major 7th intervals have function of major chord. Similarly, chords that consists of minor 3rd and minor 7th intervals have function of minor chord. Actually, nature of the chord defined with the 3rd and 7th degree of that scale. If the chords consists of major 3rd and minor 7th – it’s the DOMINANT chord. If consists major 3rd and major 7th – it’s the TONIC chord, minor 3rd and minor 7th (or major 7th) – TONIC chord.
Interesting thing is that there is only one chord that is sharing the same notes on 7th and 3rd degree of that scale, for example: C major and F#major, and those notes are E and Bb ie. Bb (=A#) and E.
The II–Ⅴ–I progression (occasionally referred to as II-Ⅴ–I turnaround) is a common cadential chord progression used in a wide variety of music genres, including jazz harmony. It is a succession of chords whose roots descend in fifths from the second degree (supertonic) to the fifth degree (dominant), and finally to the tonic. In a major key, the supertonic (triad II) is minor, and in a minor key it is half diminished seventh chord. The dominant is, in its normal form, a major triad and commonly a dominant seventh chord. With the addition of chord alterations, substitutions, and extensions, limitless variations exist on this simple formula. Need to say that diminished chord (eg. Cᴼ) has 4 different names eg. Cᴼ=Ebᴼ=F#ᴼ=Aᴼ no matter what’s the lowest note. Function of dim. chord is actually dominant flat 9. Accordingly, Cᴼ=Ebᴼ=F#ᴼ=Aᴼ –> B7(-9)=D7(-9)= F7(-9)=Ab7(-9).
Eleventh (11th) chords – consists 11th scale degree. Let’s see a few of them:
Thirteenth (13th) chords – consists 13th scale degree. Let’s see a few of them: