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Theory 101 – Key Signatures 2

Posted by Ian on March 18, 2008

Today’s lesson is a continuation of the previous lesson over key signatures.

In the previous lesson, you learned how to figure out and construct each key signature’s basic triads. Today, I

shall continue with an explination of 7th Chords , and how they fit into key signatures. With this knowledge, you

should also be able to figure out other ‘Extended’ Chords.

Before I get into explaining where 7th chords fit into key signatures, it will benifit you to have a basic

understanding of the structure of 7th chords:

A major 7th consists of a 1st, a 3rd, a 5th, and a 7th.
A minor 7th consists of a 1st, a minor 3rd, a 5th, and a minor 7th.
A dominant 7th consists of a 1st, 3rd, 5th, and a minor 7th.

Now, back to the key of C

In the previous lesson, we decided that the chords for the key of C were…

Bdim the notes were C D E F G A B

Let us construct the Cmaj7 chord.

We know that it consists of C, E, G… but what note is the 7th?

Since it is a major chord, the 7th will be a major 7th.


The major 7th in the Cmaj7 chord is B, giving us the notes CEGB to make a C7 chord.

Let’s make the Dmin7 chord….

We already know that the notes in Dmin are D F and A.

Let’s find the 7th….

D E F# G A B C#

(note… a 7th is always one note below the root on a scale)

However, as noted before, the c# is not in the key of Cmajor. You must lower it a half step to make it a C,

allowing it to fit in the key of C, giving us a minor 7th.

You now have the notes DFAC to make a Dmin7 chord.

Now, things become a little confusing when constructing the 7th chord in a key.

Let’s take a look at the construction of a Bdim7 chord (which is actually called Bmin7(b5))

A B chord consists of B D# F#. Both the D# and the F# must be lowered a half step to D and F to fit in the key of

C, yeilding a Bdim chord.

The 7th of B in its own key (the key of B) is an A#. This note is not in the key of C, so it must be lowered to an

A to fit.

You therefore have a Bmin7(b5) chord which consists of BDFA.

When the 3rd, 5th, and 7th of a chord are all forced to be lowered by a half step, it is called a min7(b5) chord

(as it has a minor 3rd and 7th, the components of a minor7th, but also has a minor 5th in it, hence the (b5) in the

chord’s name)

Like the triad construction in a key, 7th chords follow a pattern…

1- maj7
2- min7
3- min7
4- maj7
5- dom7
6- min7
7- min7(b5)

So all 7th chords in the key of C major are…


Hopefully this lesson has given you a better understanding of extended chords (specifically 7ths) and where they

fit in key signatures.

I plan on tackling minor keys in upcoming lessons, so stay tuned!


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