What chords are in what key, and why? This lesson assumes basic knowledge of the Circle of 5ths.
Part 1: Basic Triads.
Each diatonic scale has 7 different notes, which gives way to 7 possible triads for each key in music. A triad is the 1st, 3rd, and 5th notes of a scale played simultaneously to form a chord.
All chords are formed based on their respective major diatonic scale. A C chord is built on a C major scale, a D chord is built on a D major scale, etc.
There are 7 chords for each key, which correspond to the 7 notes in each key's scale. Some chords can be in more than one key - for example, a D major chord can be in the keys D, A, or G.
I'll use the key of C as an example:
The key of C includes the notes C D E F G A B C.
Each note of the scale corresponds to a scale degree as shown:
..Note: C D E F G A B C
Degree: 1 2 3 4 5 6 7 1
You can form 7 basic chords (triads) from the notes in the key of C. Each different note is the root of a different chord.
There are 3 combinations of the 1st, 3rd, and 5th notes that will be covered in this lesson. There are 3 more, but they are not included.
Major triad: 1 3 5
Minor triad: 1 b3 5
Diminished triad: 1 b3 b5
Your first chord will be a C chord, because C is the first scale degree. Now, since this is a C chord, it will be based on the C major diatonic scale. Take scale degrees 1 3 5 as shown below:
..Note: C D E F G A B C
Degree: 1 2 3 4 5 6 7 1
........*...*...*
This gives you notes C, E, and G. Since all 3 of those notes are in the key of C, you do not have to modify them to fit, and you have a major triad (1 3 5). So your first chord is C major.
The second chord will be a D chord, because D is the 2nd scale degree. It's based on the D scale, which is D E F# G A B C# D. Now, take 1 3 5 of this scale:
..Note: D E F# G A B C# D
Degree: 1 2 3 4 5 6 7 1
........*...*....*
This gives notes D F# A. This presents a problem - F# is not in the key of C! In order to keep this chord in key, we have to flat the F# (lower it by 1/2 step) down to F natural. This gives D F A, which is scale degrees 1 b3 5 of the D major scale. 1 b3 5 is the formula for a minor triad. Therefore, your second chord is D minor.
The seventh chord will be a B chord, because B is the 7th scale degree. It's based on the B scale, which is B C# D# E F# G# A# B. Now, take 1 3 5 of this scale:
..Note: B C# D# E F# G# A# B
Degree: 1 2 3 4 5 6 7 1
........*....*....*
This gives notes B D# F#. D# (3) and F# (5) are not in the key of C, and must be flatted to D (b3) and F (b5), respectively. This gives us scale degrees 1 b3 b5, which is the formula for a diminished triad.
Based on these examples, you can figure out the rest of the chords. However, they always follow a pattern:
1 - major
2 - minor
3 - minor
4 - major
5 - major
6 - minor
7 - diminished
By applying this pattern, you can quickly figure out that the chords in the key of C are:
Cmaj
Dmin
Emin
Fmaj
Gmaj
Amin
Bdim
All the notes contained in the above chords will be in the key of C.
This pattern works for any of the keys in the Circle of 5ths. It does not, however, cover any scales that are not the major scale (such as the harmonic minor scale, for example. That has its own pattern of chords).
Part 2: Extended chords.
Okay, so you've got the basic triads down? Great! Now on to extended chords. First, you must learn the formulas for the 4 types of 7th chords.
......Major 7th: 1 3 5 7 - Abbreviation: maj7
......Minor 7th: 1 b3 5 b7 - Abbreviation: min7
...Dominant 7th: 1 3 5 b7 - Abbreviation: 7, dom7
Minor/Major 7th: 1 b3 5 7 - Abbreviation: min/maj7
Now, let's return to our first chord. We know it's a major chord from Part 1. We can now figure out what type of 7th chord it is using the same method.
..Note: C D E F G A B C
Degree: 1 2 3 4 5 6 7 1
........*...*...*...*
Your notes are C E G B, all in the key of C. No changes are needed to the notes, and so this is a maj7 chord.
Our second chord was a minor chord in Part 1. Let's take it to the next level, a 7th chord.
..Note: D E F# G A B C# D
Degree: 1 2 3 4 5 6 7 1
........*...*....*...*
The notes are D F# A C#. F# (3rd) and C# (7th) are not in the key of C, and must be flatted on down to F natural (b3rd) and C natural (b7th). Therefore, your scale degrees for this chord are 1 b3 5 b7. This gives us a min7 chord.
Our 5th chord is a G chord - let's find the 7th.
..Note: G A B C D E F# G
Degree: 1 2 3 4 5 6 7 1
........*...*...*...*
Our notes are G B D F#. F# (7th) must be flatted to an F natural (b7). Our scale degrees are 1 3 5 b7, which is the formula for a dominant 7th chord. Our 5th chord is G7!
The seventh chord is a Bdim chord as shown in Part 1. Extending this chord, we find that it is a min7(b5) chord.
..Note: B C# D# E F# G# A# B
Degree: 1 2 3 4 5 6 7 1
........*....*....*.....*
This gives notes B D# F# A#. The D#, F#, and A# are all flatted 1/2 step to give degrees 1 b3 b5 b7. This is the formula for a min7(b5) chord, also known as a half diminished chord.
Using the same method you can figure out the other chords. They also follow a pattern. That pattern goes as follows:
1 - maj7
2 - min7
3 - min7
4 - maj7
5 - dom7
6 - min7
7 - min7(b5)
And, as you may have guessed by now, the chords in the key of C are:
Cmaj7
Dmin7
Emin7
Fmaj7
G7 OR Gdom7 (they are the same chord)
Amin7
Bmin7(b5)
That's all for now. Feel free to PM me if you have questions, or visit the Musician Talk forum!
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