1 Music Theory – 1G Extensions and Alterations

In this lesson we will learn:

To understand extensions

To understand alterations (altered extensions)

Examples of extensions and alterations

Four note chords (1-3-5-7) cover all the necessary information to provide an adequate harmony. If you change any one of those notes you actually are changing the harmony the chord represents and thus the possible scales that the chord underlines. However you can “extend” the harmony without changing the underlining chord. You do this by continuing stacking thirds. This creates a new numeric pattern for chords 1-3-5-7-9-11-13. The 9,11 and 13 of a chord are called extensions (aka. tensions or tension notes). They are the same as the 2,4 and 6 of the scale (look at the picture below). You can also alternate extensions by raising or lowering them by a semitone. The altered extensions are called alterations. The possible alterations are the b9,#9,#11 and b13. I have provided audio examples for each of the extensions and alteration so you can get a “feel” of what they sound like. All examples are in the key of C.

Additional information available!

Note: I want my lesson to be very approachable and straight to the point. That is why I have put all the “deep” information under “1G Extensions and alterations”. There is background information regarding chord-scale relationship, terminology, when to use specific extensions, naming chords with extensions/alterations and a ton of other important information regarding the subject. So if you want more specific information regarding extensions and alterations please click on the button below. 

Understanding extensions 

In the illustration above you can see the general idea behind extensions (also called tensions or tension notes). They “fill in” the chord tones with the notes of the rest of the (major) scale. Since extensions are usually played on top of the foundation which is the chord tones (1-3-5-7) their respected numbers are over 7. If the notes are played in one octave together with the chord tones then the numbers are under 7. The picture shows clearly the relationship between scales and chords. Scales are build in seconds (vertically – 1,2,3,4,5,6,7,8) and chords are build in thirds (horizontally – 1-3-5-7-9-11-13).

The extensions are formed by continuing to stack thirds on top of each other after the 7th chord tone. So far we have had 1-3-5-7 chords. Now we extend it to nine chords (1-3-5-7-9), eleven chords (1-3-5-7-9-11) and thirteen chords (1-3-5-7-9-11-13). If we stack one more third we go back to the root so there is no “fifteen chord”. The thirteen chord is thus a “fully extended” chord. The 9,11 and 13 are the extensions to the four note chords. Each four note chord has specific extensions that are based on the scale that the chord represents. Note that a chord can represent several different scales simultaneously. Check out “Additional Information” for more details.

A “fully extended” Cmaj13 chord.

Note: The illustration above shows the “base” of the chord (1-3-5-7 RED) and the extensions (9-11-13 BLUE). You can see how the extension notes “fill in” the remaining notes of the (C major) scale.



The 9 is the second note of the scale played an octave higher. Basically the 2 and the 9 are the same note. It is often used as a melody note or even as a target note for ending lines and patterns while improvising. It creates a rather consonant sound. I have provided some common examples of four note chords using the 9 as a melody note. Listen to how the 9 produces a different effect for each chord. I have also provided an example scale for reference. Please note that several scales are possible for each chord. 


Cmaj9 (C major scale)


Cm9 (Bb major scale, starting on C)


C9 (F major scale, starting on C)


Cø#9 (Eb mel. minor scale, starting on C)


CmMaj7 (C mel. minor)


Co9 (C W/H diminished scale)



The 11 is much less used compared to the 9 and the 13. It creates the most “unresolved” sound and is thus used mostly with chords that need resolving such as dominant sustained (sus) chords. To be able to play it with other types of chords it is usually raised to a #11 or it is omitted altogether. We will cover that when we look at alterations. It is mostly used with minor and dominant sustained (sus) chords where the 11 replaces the 3 (in other words the 3 is usually omitted). 


Cm11 (Bb major scale, staring on C)


C11 / C7sus (omit 3) / Bb/C7 (F major, starting on C)


Co11* (C altered)

*This is a very specific sounding voicing that you might have heard. It is quite “exotic” and rare. There is no “proper” way to write that chord. Just listen to the example.



The 13 is the same as the 6. But if the chord has a 13 it means it actually must have a 7 in it as well. Both 13 and 6 chords are commonly used. It has a similar “resolving” sound as the 9. It still has less adaptations than the 9. 


Cmaj13* (G major, starting on C)

*Cmaj13 has #11 in it instead of 11. The 4 (=11) is an avoid note. Look at “Additional Information” for more details.


Cm13 (Bb major scale, starting on C)


C13 (F major scale, starting on C)

C13 sometimes has a #11 in it. Sometimes it is omitted. The 11 is usually omitted.


Understanding alterations

Alterations are mostly used with dominant chords. We can alternate each extension of a (dominant) chord. Some extensions can be raised and lowered a semitone , some can be just raised a semitone and some can be just lowered a semitone. That is because an alteration will be an alteration only if it does not become a chord tone (1-3-5-7) or a new extension by altering it. This will become clear when we go through each alteration separately. You can also look at the diagram below for reference.

A “fully extended” Cmaj13 chord with all possible alterations.

9 > #9 / b9

The 9 can be lowered to a b9 (flat nine) or raised to a #9 (sharp nine). In the key of C the ninth note (also the second) is a D. So a b9 would be a Db and a #9 would be a D#. Both notes are not part of the four notes that determine the harmony (1-3-5-7) in C major so the alterations are valid.



C7b9* (F harmonic minor, starting on C)

Sometimes has a 13 or a #11 added as well (depending on the voicing)



C7#9* (Ab major, starting on C)

*Sometimes has a b13 added as well (depending on the voicing)


11 > #11

The 11 can only be raised to a #11 (sharp eleven). In the key of C it is a F# and not a chord tone. If we lower it then it becomes the 10th note (thus also the 3rd note) of the scale. In the key of C that is an E and because it is the 3 (out of 1-3-5-7) it cannot be an alteration. 


Cmaj7#11* (G major, starting on C)


C7#11* (G mel. minor, starting on G)

*Usually has the 13 added as well


13 > b13

The 13 can only be lowered to a b13 (flat thirteen). In the key of C it is Ab and not part of a chord tone. If we raise it then it becomes the #13 (thus also the b7) of the scale. In the key of C that is an A#=Bb and because it is the b7=7 (out of 1-3-5-7) it cannot be an alteration. 

Note: In theory a chord can have both a (major) 7 and a b7=#13 but it is considered an avoid note and creates great tension. The problem with #13 is that it might be interpreted as changing the quality of a chord to a dominant seven because it is the same note as a b7.


C7b13* (Db major, starting on C)

*Usually has a b9 or #9 added as well (depending on the voicing)


The 6/9 chord

There is a specific chord voicing that is usually written as a 6/9 voicing. It uses both the 9 and 13 extension. However because the 13 replaces the 7 in that chord is becomes a 6 (the chord has no seven). It is most commonly used in static (=“stable”) sounding chords such as tonics (“Root” chords). The following audio examples show two three chord progressions both ending on the C6/9 and Cm6/9 chord respectively.  


 C6/9 (C major)

2-5-1 in C major

Cm6/9 (C melodic minor)

2-5-1 in C minor

Playing and voicing chords with extensions and alterations in practice

In practice playing the chords “just” as stacked thirds is not recommended as it might sound predictable and dull. Players usually rearrange the order of notes and omit or double some notes to make it more aurally pleasing. As a general rule the 1 (=Root) (sometimes played much lower or ommited), 3 and the 7/6 are played in the lower register while all extensions and alterations (and chord tones) are played in the upper register. This is the premise of chord voicing and we cover that in Chapter 2. Here are two examples of how one might voice a Cmaj9 and C7#9b13 chords. 

Example Voicing Cmaj9
Example Voicing C7#9b13 (Upper Structure)

Reading and playing chords in practice

Most composers of popular music and jazz want to convey as much information as possible with using as little space as possible. That is why mostly chords will be written as either plain triads or simple four note chords. It is up to the player to “fill in” the appropriate extensions and alterations depending on the mood, style and need of the the situation and/or composition. That is why it is good to learn all the possible extensions and alterations so you know what should be available at your disposal if a situation arrises that you have to accompany yourself or somebody else.

Generally, as a player who produces harmony, you are given the freedom to “arrange” any piece you play according to your taste by adding extensions and alterations to the chords and you will be given just the “bare bones” of the harmonic structure to work with. An exception to this rule are specific arrangements that are usually done for combos and larger ensembles where everyone has to play the exact extensions and alterations (not in the same order. Once again this will become more apparent the more time we spend on the subject. 

(c) Sibil Yanev 2019