7 Species of Seventh Chords Worth Knowing

If you play natural and harmonic major and minor scales, you can build the following species of seventh chords based on the 3 consecutive triads. These seventh chords are systematized according to what kind of seventh is between the two outer notes (major - 5.5 steps, minor - 5 steps, diminished - 4.5 steps) and what kind of triad is produced out of the 3 lower notes (major - M3-m3, minor - m3-M3, diminished - m3-m3, augmented - M3-M3). Below is the list of 7 species of seventh chords you may find worth spending the time with.

1. Minor Major Seventh Chord (C-E-G-Bb):
Structure: Minor 7th between the outer notes and major triad. 
Function: Dominant seventh chord (natural major and harmonic minor).

2. Minor Minor Seventh Chord (C-Eb-G-Bb):
Structure: Minor 7th between the outer notes and minor triad. 
Function: Seventh chord of the 2nd and the 6th scale degrees (natural major), Tonic seventh chord and Subdominant seventh chord (natural minor).

3. Minor Diminished Seventh Chord (C-Eb-Gb-Bb):
Structure: Minor 7th between the outer notes and diminished triad.
Function: Seventh chord of the 7th scale degree (natural major), seventh chord of the 2nd scale degree (natural minor and harmonic major).

4. Diminished Seventh Chord (C-Eb-Gb-B double flat):
Structure: Diminished 7th between the outer notes and diminished triad.
Function: Seventh chord of the 7th scale degree (harmonic major and harmonic minor).

5. Major Major Seventh Chord (C-E-G-B):
Structure: Major 7th between the outer notes and major triad.
Function: Tonic seventh chord and Subdominant seventh chord (natural major), seventh chord of the 3rd scale degree and seventh chord of the 6th scale degree (natural minor).

6. Major Minor Seventh Chord (C-Eb-G-B):
Structure: Major 7th between the outer notes and minor triad.
Function: Subdominant seventh chord (harmonic major), Tonic seventh chord (harmonic minor).

7. Major Augmented Seventh Chord (C-E-G#-B):
Structure: Major 7th between the outer notes and augmented triad.
Function: Seventh chord of the 6th scale degree (harmonic major), seventh chord of the 3rd scale degree (harmonic minor).

Study these 7 species of seventh chords and try to play and/or write them in any key and from any note you want. By doing so they will become your own and you will start to notice them in real music that you play. Moreover, you will be able to use them in your own improvisations and/or compositions.