How to Build Chord Extensions and Alterations

It’s important that you understand that all 7^th chords can be extended and altered. This is not only important to comprehend if you play a chordal instrument, like a piano or guitar, but it’s also important to know as an improviser.

Why you may ask? Throughout your jazz studies, you will most surely come across chords labeled, for example, as a C7(#11) or Bbmin11. You’ll want to know how to construct these chords and how they may influence your note choices as an improviser.

In today’s lesson, I’m going to go over how extensions and altered extensions work over 7th chords, and which ones you can use. Much of what is covered today comes from our eBook Zero to Improv, which teaches you how to become a great jazz improviser from the ground up.

First things first, let’s go over some definitions to get off on the right foot!

What’s a chord extension?

Chord extensions are essentially chord tones that are added above the basic 7^th chord structure (R-3^rd-5^th-7^th). The possible extensions are the 9^th, 11^th, and 13^th. These extensions don’t replace the R-3^rd-5^th-7^th but are added in addition to achieve a desired sound. However, in some cases, an extended chord may exclude a basic chord tone to avoid dissonance.

The easiest way to understand chord extensions is to think of them as the notes in between the basic structural chord tones: the 2^nd, 4^th, and 6^th.

The 9^th is the same as the 2^nd, just up an octave.

The 11^th is the same as the 4^th, up an octave.

The 13^th is the same as the 6^th, up an octave.

If that doesn’t quite make sense right away, hopefully, this visual will help you see what I am talking about.

Which extensions can you use on 7th chords?

Let’s go through which extensions you can use on different kinds of 7^th chords. This is especially important to understand for composing and for chordal accompaniment instruments.

Note that these are un-altered extensions. We will go over altered in a second, but it’s important to mention because in some cases an altered extension can be used on a chord that would not use an un-altered extension. This will become clear.

9th

The 7^th chords it can be added to: Major, dominant, minor, half-diminished.

Formula: R-3^rd-5^th-7^th-9^th

Example: Cmaj9

Note: If you know the formulas for all of the 7^th chord qualities, you can add a 9^th to any of these chords. Build the basic 7^th chord first and then simply add the 9^th on top. Easy!

11th

The 7^th chords it can be added to: minor, half-diminished, diminished

Rule: the 11^th can be added to chords with a b3 in it. Otherwise, the 11^th would clash with the major 3^rd.

Formula: R-3^rd-5^th-7^th-9^th-11^th

Example: Cmin11

13th

The 7^th chords it can be added to: major, dominant, minor.

Formula: R-3^rd-5^th-7^th-9^th-13^th

Example: C13

Note: 13^th chords usually do not include the 11^th in the chord.

Now that we’ve covered chord extensions let’s talk about altered chord tones. Here’s a good definition:

What’s an altered chord tone?

An altered chord tone is any functioning chord tone (structural or an extension) that is raised or lowered by a half step to achieve a desired effect. This is often done for voice leading purposes and to achieve some kind of tension and release.

Altered chord tones over Major 7ths

Possible alterations: b5, #5, #11, b13.

Note: While the 11^th is not used in a major 7 as an un-altered extension, it is used as an altered chord tone, specifically a #11.

Additional Note: The b13 is not very common.

For the sake of being thorough, I think it’s important to demonstrate what the difference between a b5 and a #11. The b5 is technically not an extension, it’s a chord tone. But If you think about it, they are the same note. In the key of C that would be a Gb/F#. Take a look:

If it isn’t clear to you, the Cmaj7(b5) simply takes the existing perfect 5th and flats it. But the Cmaj7(#11) keeps the perfect 5th and instead adds the extended #11th on top of the chord.

Altered chord tones over Dominant 7ths

Possible alterations: b5, #5, b9, #9, #11, b13.

Note: The dominant 7 chord has the most alterations possible. Also, the same as it was with the major 7, the 11th can be used with the dominant 7 as an alteration (#11).

One important chord to understand when it comes to dominant 7ths is the alt chord. If you ever see on a piece of sheet music “C7alt” that just means that some or all of the extensions are included in the chord and altered. Jazz musicians can choose to outline all of them, whether playing a chord or improvising, or picking and choose which ones are included.

It may be a lot of notes crunched up together on the staff, and not the easiest to read, but go ahead and try to identify all of the altered extensions in this notated C7alt chord.

Altered chord tones over Minor 7ths

Possible alteration: Major 7th

Note: There are those that would alter extensions on a minor 7 chord, but in general it’s uncommon and unconventional.

Some would not consider altering the b7 to a natural 7 in a minor 7 chord to be an “alteration.” They would consider it a different chord altogether. Technically it’s just a minor triad with a major 7 added on top. However, I like to think of it as an alteration.

And that’s it! Those are the only 7th chord qualities with alterations. Some of this may seem very un-musical to you, and you would be correct. This is jazz theory 101. But trust me, knowing this stuff and being competent will give you a serious leg up on your jazz improvisation!

Understanding these chords is Step 1. Step 2 is being able to recognize these by ear.