Chord Extensions - Music Theory Lesson

Seventh Chord Construction

Before delving into chord extensions, here is a quick recap about seventh chord construction. First, you need to know that seventh chords (four note chords) are constructed based on triads (three notes), built by stacking thirds. These triads can be major, minor, diminished or augmented. Here they are:

Triads

• Major Triads are built with : root (1), major third (3) and perfect fifth (5). 

• Minor triads are built with :  root (1), minor third (b3) and perfect fifth (5).

• Diminished triads are built with :  root (1), minor third (b3) and diminished fifth (b5).

• Augmented triads are built with :  root (1), major third (3) and augmented fifth (#5).

Seventh Chords

To build a seventh chord, just add a supplementary note on top of the previous triads, thus providing ten types of seventh chords built this way:

• Major 7 (major triad + major seventh). Giving the formula root (1) - major third (3) - perfect fifth (5) - major seventh (7). 

• Dominant 7 (major triad +  minor seventh). Giving the formula root (1) - major third (3) - perfect fifth (5) - minor seventh (b7). 

• Augmented seventh (augmented triad + minor seventh). Giving the formula root (1) - major third (3) - augmented fifth (#5) - minor seventh (b7). 

• Augmented major seventh (augmented triad + major seventh). Giving the formula root (1) - major third (3) - augmented fifth (#5) - major seventh (7). 

• Minor 7 (minor triad + minor seventh). Giving the formula root (1) - minor third (b3) - perfect fifth (5) - minor seventh (b7). 

• Minor major 7 (minor triad + major seventh). Giving the formula root (1) - minor third (b3) - perfect fifth (5) - major seventh (7). 

• Half-diminished (minor triad + minor seventh). Giving the formula root (1) - minor third (b3) - diminished fifth (b5) - minor seventh (b7).  

• Diminished 7 (minor triad + diminished seventh). Giving the formula root (1) - minor third (3) - diminished fifth (b5) - diminished seventh (bb7). 

What's A Chord Extension?

Notice that there are two main types of extensions (or tensions) : Unaltered Chord Extensions (aka natural chord extensions) and Altered Chord Extensions. 

Unaltered Chord extensions are the chord tones added beyond the seventh of a basic seventh chord (1- 3 - 5 - 7). There are three unaltered chord extensions, the 9th, 11th and 13th that are respectively the same as the 2nd, 4th and 6th just between each chord tone.

Altered chord extensions are any chord tone (or extension) raised or lowered by a half-step to create some tension or just use for voice leading. These altered chord tones are : b9, #9,  #11 and b13, respectively b2, #2, #4 and b6 up an octave. They are all present in a same scale named "The Altered scale", seventh mode of the melodic minor scale.

Now we will see what extensions can be applied to the ten types seventh chords previously listed in this lesson. Let's start with Dominant 7th chords (1 - 3 - 5 - b7)

Dominant Seventh Chord Extensions

Dominant 7 chords (7) are built with root (1), major third (3), perfect fifth (5) and minor seventh (b7),  they are those that accept the most extensions :

• Natural Extensions : ninth (9), thirteenth (13).
• Altered Extensions : Flat ninth (b9), sharp ninth (#9), sharp eleventh (#11) and flat thirteenth (b13).

Here are some examples in the key of C.

If you encounter the "alt" label just after the chord name (ex. C7alt), it means that the chord contains several or all the possible altered extensions as b9, #9, #11, b13.

Concerning the 11th, it must be used with caution because it clashes with the major 3rd (it is is the most often used with minor chords). The most common way to use it is in suspended fourth chords (where the third is left out). 

Notice that the #11 should not be confused with the b5. Although these two notes are the same, the b5 is not considered an extension but a chord tone. Theoretically, a 7b5 chord is a dominant 7 chord with a flat fifth (1 - 3 - b5 - b7) whereas a 7#11 chord must implie a perfect fifth and a ninth, giving the formula (1 - 3 - 5 - b7 - 9 - #11).

Minor Seventh Chord Extensions

Minor seventh chords (min7, m7) are made of a root (1), minor third (b3), perfect fifth (5) and minor seventh (b7). They can be enriched by the natural extensions 9, 11 and 13. Generally, they don't accept altered extensions, however it is possible to find m7b9 or m7b13 chords.

Major Seventh Chord Extensions

Major seventh chords (Maj7,  ∆, M7) a built with root (1), major third (3), perfect fifth (5) and major seventh (7). They can be extended with 9, 13 and also altered with the #11.

Half-Diminished Chord Extensions

Minor seventh flat five chords aka half-diminished (Ø, m7b5) are built with root (1), minor third (b3), diminished fifth (b5) and minor seventh (b7). They can be enriched with the 9 and 11.

Diminished 7 Chords

Diminished seventh chords (dim7, °7) are made of root (1), minor third (b3), diminished fifth (b5) and diminished seventh (bb7). The possible extensions are 9, 11, b13. Note that the b5 is also the #11.

Augmented Major Seventh Chord Extensions

Augmented major seventh chords (maj7#5, ∆#5) are built with root (1), major third (3), augmented fifth (#5) and major seventh (7) The tensions are the 9 and #11.

Minor Major Seventh Chord Extensions

Minor major seventh chords (minMaj7, m∆) are composed of a root (1), minor third (b3), perfect fifth (5) and major seventh (7). The possible tensions are the ninth (9), sharp eleventh (#11) and thirteenth (13).

Dominant Seventh Flat Five Chord Extensions

Dominant 7b5 chords (7b5) are built with root (1), major third (3), diminished fifth (b5) and minor seventh (b7). They accept the following tensions : flat ninth (b9), ninth (9), sharp ninth (#9), flat thirteenth (b13) and thirteenth (13).

Dominant Seventh Sharp Five Chord Extensions

Dominant 7#5 chords (7#5) are made of a root (1), major third (3), augmented fifth (#5) and minor seventh (b7). They accept the following tensions : flat ninth (b9), ninth (9), sharp ninth (#9) and sharp eleventh (#11).

Suspended Dominant Chord Extensions

Suspended Dominant chords don't have any third. They are built of a root (1), perfect fourth (4), perfect fifth (5) and minor seventh (b7). The tensions that can be added are : flat ninth (b9), ninth (9) and thirteenth (13).