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Chords of Major Keys - Extended and Secondary Dominant Chords and Mode Mixture

Updated on December 20, 2015

This lesson was written to complement a previous lesson called Chords of Major Keys - Major, Minor and Diminished triads. In that lesson we explored the naturally occurring triads that are formed by combining notes of the major scale. Also included was a very important four-note chord, called the dominant 7th chord, formed by adding a note to the triad built on scale note 5 (the dominant) .

In this lesson, we'll take a look at more chords built by adding notes to all the other triads, as well as some chords that technically don't belong completely to the major scale, but are nevertheless commonly found in songs written in major keys.

Keep in mind that we're showing how the chords are derived from their notes' positions in the scale. In practice any notes can be doubled and played at any octave.

The scale of C major written over two octaves
The scale of C major written over two octaves

Seventh Chords

Again, using the scale of C major as our example, we can select the same notes as we did with the triads, plus an extra note that, like the triads' notes, lies a 3rd above the last note.

7th chords formed from notes of the C major scale separated by intervals of thirds
7th chords formed from notes of the C major scale separated by intervals of thirds

Look at the first chord: C major 7th. As you can see, it consists of the notes, C, E, G & B. The first three notes, C, E & G, make the triad called C major, exactly as was explained in the other lesson. By selecting another note a 3rd above the last chord note of the C major triad, we can complete the new chord. As this new note, B, is seven scale degrees above the first note or root, the interval between them is a 7th, and so the chord will be a type of 7th chord. The note B is exactly eleven semitones (or half steps) above the root, which tells us that that particular interval of a 7th is a major 7th. That's where the chord gets its name.

The second chord, D minor 7th, is formed in the same way; that is, we add another scale note (a 3rd above the last note) to the triad. This time, the new note, C, seven notes above the root, D, is only ten semitones or half steps above the root. That makes the interval between them a minor 7th. However, the minor part of the chord's name (D minor 7th) refers to the chord being based on a minor triad not to the minor 7th interval that the chord contains. This is a source of confusion, but the following should make it clear:

C (major) + major 7th = C major 7th (The first major is assumed).
D minor + (minor) 7th = D minor 7th (The second minor is assumed).

All the other 7th chords are formed in the same way.

7th chords add extra flavour and can intensify the harmonic function of the chord. For example, the triad built on scale note 5, (the dominant chord, conventionally referred to in Roman numerals as V), has a mild tendency, or at least an expectation, to proceed to the tonic chord. Converting the chord to a 7th chord (V7) gives it a sense of greater urgency because it contains a dissonance best resolved by proceeding to the tonic chord. If you have an instrument handy, you can hear the difference by comparing them. In our example in the key of C major, the chord progressions to compare would be:

G major followed by C major (V - I) compared with G7 followed by C major (V7 - I).

If you play them, you can hear how G7 leads to C major more strongly than G major leads to C major.

Extended chords

C major scale over 2 octaves showing the 'chain' of 3rds that make extended chords of the key, C major
C major scale over 2 octaves showing the 'chain' of 3rds that make extended chords of the key, C major

By continuing the process of adding notes a 3rd above the last one, we get new types of so-called extended chords, named after the number of notes between the chord's root and the last note used. These include a variety of 9th chords, 11th chords and 13th chords. 13th chords are as far as we can go because if we add another 3rd we come back to where we started. You won't hear any number greater than 13 mentioned in relation to chords.

To summarise how we reached this position,

Start on any scale note and select alternate notes (i.e., notes separated by intervals of a 3rd). Think of your starting note as note 1 and count from there. Selecting three notes this way gave us a series of major and minor triads and one diminished triad (as explained in the earlier lesson mentioned above). Selecting four notes gave us a series of 7th chords as explained above. Continuing the process to select five, six and seven notes separated by 3rds results in a wide variety of extended chords. If you want to understand how the names of chords relates to the actual notes they contain, have a look at my chord construction article.

5 notes (1 3 5 7 9)
This produces a variety of 9th chords.

6 notes (1 3 5 7 9 11)
This produces various 11th chords - In practice the 3rd is often omitted if it clashes with the 11th of the chord.

7 notes ( 1 3 5 7 9 11 13)
This is the final extension possible and produces a wide variety of 13th chords. In practice some notes are usually omitted, such as the 11th because of its strong dissonance with the 3rd, and some notes, such as the 9th and 5th, are optionally omitted as they aren't essential to the sound or function of the chord.

Added note chords

If we omit the 7th from any of the extended chords, they are called added note chords.

For example:
Added 9th chords consist of scale notes 1 3 5 & 9 (in relation to the root).

There's no 7th in the chord. It's simply a C major chord with the 9th (or 2nd) scale note added. It's labeled as C add9.
Note that you may also see it as C add2. That's because the note D hasn't been arrived at by stacking 3rds as was the case with the extended chords, so there's no real reason to call it 9, when it's actually just the 2nd scale note above the root that's been added to the chord. However, probably most people (including music publishers) prefer to call it add9 rather than add2.

Common added note chords include 6th chords (the word added isn't used with them) consisting of the triad plus the sixth scale note above the root, e,g., C6 consists of notes C E G & A (1 3 5 & 6). Others are also possible, following the same logic, e.g., add 4 (or add 11). 6 add9, (aka six-nine) usually written as 6/9. That contains scale notes (1 3 5 6 & 9).

Suspended chords are decorative chords similar to added note chords except that the suspended note is the 4th or 2nd scale note (in relation to the root) and it replaces the 3rd of the chord.

Suspended 4th chords (sus 4 or simply sus) contain scale notes 1, 4 & 5 (in relation to the root).
C sus4 = C F & G
G sus 4 = G C & D


Suspended 2nd chords (sus 2) contain scale notes 1, 2 & 5 (in relation to the root).
C sus 2 = C D & G
G sus 2 = G A & B

Out of key chords

Many songs in major keys use notes and chords that are outside of the key.

Blues

In blues music, the tonic (I) and subdominant (IV) chords are straight forward seventh chords rather than the major 7th chords you would expect (see the 7ths table above).

Note that what I referred to as the straightforward 7th chords, (As opposed to major 7ths or minor 7ths) are commonly known as dominant 7ths, even if they're not actually on the dominant scale degree. Don't confuse "a" dominant 7th chord with "the" dominant 7th chord.

In a blues song in the key of C major- Chord I (the tonic) will be C7 (consisting of notes C, E, G & B flat) and chord IV (the subdominant) will be F7 consisting of notes (F, A C & E flat). Those notes B flat and E flat don't belong to the key of C major, but are necessary for the bluesy sound.

Mode mixture

This refers to borrowing chords from another (parallel) mode (i.e., one that shares the same tonic). Usually this is the parallel minor key. So a song in C major may borrow chords from the key of C minor, such as B flat major and F minor. (Chords in minor keys are formed in exactly the same way as in major keys, i.e combining scale notes separated by 3rds).

Secondary dominants

Although the tonic chord is normally heard as the main chord that the others relate to, it's possible to tonicise any other major or minor chord (i.e., make it sound like the tonic chord of a new key).This is done by preceding it with the chord that would be the dominant 7th of that new key.

Look at this common progression in the key of C major

C - Am - Dm7 - G - G7 - C

In Roman numerals, this would be shown as: I - vi - ii7 - V - V7 - I

If we replace D minor 7th with D7, we're using a chord that is outside of the key of C major (because D7 contains the note F sharp, which isn't in the C major scale).

C - Am - D7 - G - G7 - C

That D7 chord however, happens to be the dominant 7th chord of the key of G major and placing it just before the chord G major, means it will have a strong pull to it and will force us to hear the G major chord in a different way. We'll hear it as the tonic chord in the key of G major, sounding stable and balanced, rather than it's real job as the dominant chord of C major. This is a common device for actually changing key (modulating). In our example above, the D7 briefly tonicises the G major chord, but the next chord G7, (which is the dominant 7th of C major but foreign to the key of G major) cancels that effect and reasserts C major as the true key. The Roman numerals for that progression are:

I - vi - V7/V - V - V7 - I

V7/V has replaced chord ii. It means: the dominant seventh chord of the key that corresponds with scale degree V.

Conclusion

I hope this lesson in conjunction with the one dealing with simple triads has explained the derivation of the most common chords that you are likely to encounter in real music in major keys. Songwriters and composers are completely free, of course, to choose any chords they wish, but most of the time it will be the chords shown here that are chosen because of how they relate to the intended key of the music.

If you'd like to read about chords of minor keys, visit my article:

Chords of Minor Keys

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    • manthy profile image

      Mark 5 years ago from Alabama,USA

      Very Accurate and easy to read, I love this and look forward to more.

    • chasmac profile image
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      chasmac 5 years ago from UK

      Thanks Manthy - glad you liked it.

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