This tutorial will look at what happens when we continue to stack thirds, following the principles of tertian harmony. We have already dealt with stacking up to four thirds and we have created many chord types, including but not limited to

• Major
• Minor
• Diminished
• Augmented
• Dominant Seventh
• Major Seventh
• Minor Seventh
• Half diminished Seventh
• and many more types of seventh chord

What comes next? If we continue to stack thirds we can create further chord extensions

• 9^th
• 11^th
• 13^th

This non members tutorial will look at how we create 9^th chords. If you would also like to see how we create 11^th and 13^th chords and understand more about this process then please consider subscribing to silver membership where you will have access to the complete tutorial, along with access to all other complete tutorials, downloadable copies of all of the tutorials and reference sheets to aid in constructing your own composers reference kit or check out the full tutorial in the Buy Me A Coffee Shop.

Chord extensions

Chord extensions are a powerful way to add color, depth, and emotional nuance to harmony. They appear everywhere—from jazz and R&B to film scores, pop ballads, and orchestral writing.

In order to understand them we just need to continue to apply the process we have already discussed in previous tutorials, that of stacking thirds.

Ninth Chords

As we saw previously, we start with a root note and add a third, then another third to create the basic triad. If we continue and add another third we get a seventh chord. This is the foundation of our ninth chords. We can take all of our different seventh chords and add a further third, which can be either a major or a minor third.

Let’s look at the example of a dominant ninth chord, C^9, stacking thirds to obtain the notes. A C⁹ chord is built from a dominant seventh chord adding a further major third to it to give us the notes C – E – G – Bb – D and is composed of thirds thus

• Major third
• minor third
• minor third
• Major third

As we have four different triad types, and we are adding two further stacked thirds which can each be of major or minor quality there are numerous 9^th chords that can be created. Below is a table detailing the main ninth chords that we use

As we saw in our tutorial on intervals the compound interval of a ninth is a simple interval of a second but an octave higher.

Why Extensions Matter: The Emotional Palette

Each extension adds a distinct emotional flavor. They have the same basic form as the simpler triads but create a different mood or ambiance. They are highly used in jazz for this effect and to add harmonic interest to an otherwise standard chord progression.

The 9th – Colorful, open, lyrical

The 9th adds a sense of:

• openness
• lyricism
• gentle brightness
• emotional lift

The Major 9ths feel lush and romantic.
The dominant 9ths feel soulful and bluesy.
The minor 9ths feel introspective or mysterious.

Why learning extensions is a good thing

Understanding chord extensions helps us:

• write richer harmonies
• voice chords more creatively
• improvise with more color
• analyze jazz and film music
• create emotional nuance
• break out of triad‑only thinking

They’re one of the fastest ways to elevate our harmonic vocabulary.