Hi,
I'm learning the Dominant 7ths in C 2 octaves, but don't get why it starts on a G, and how it is built.

Any ideas?

Thanks! Tim.

Easy.
Think about the name Dominant... that's the 5th note of a scale, isn't it?
So if we have Dominant 7th of C Major, it would start on the 5th note of C major, which as you have it, is G.

Then where does the 7th come in? Well, if you count from the G, then the 7th is an F.

So there you have it, a domninant chord or triad in C major, GBD, and a 7th from the G, so you get GBDF.


In the same way we can work out the dominant 7th of other keys.. like G major.
Dominant of G major = D
Dominant chord = D F# (G major has F#, remember) A
7th of the dominant (D) = C
Hence dominant 7th of G major = DF#AC

Hope that helps

W-O-W

I didn't know that I didn't know it but now I am glad that I do!

But why is it called the Dominant 7th of C, rather than a variation on G7?

The dominant 7th of C is a G7. Calling it a dominant 7th of C shows the relationship to C major. This is important as then other chords in a passage of music can be analysed in relation to C major or any other key too.

It is not a variation on G7. It IS G7. It is also the dominant seventh chord in the key of C major.

Each note of a scale has a name:

In C major:
C = tonic
D = supertonic
E = mediant
F = subdominant
G = dominant
A = submediant
B - leading note

When building a chord on any step of the scale, the chord takes the same name as the step of the scale it is built on.

So, a chord of F major would be the subdominant chord in C major. It is also the tonic chord in F major, and the dominant chord in Bb major. However, no matter what its various names in different keys, it remains a chord of F major. You might ask why the same chord needs all these names - this will become clear when/if you begin to study harmony.

The dominant seventh is the dominant chord with a seventh on top. In the key of C major, this is a G7 chord.

I hope this is a little clearer now. It IS confusing when you first start.

Yes, and you'll realise later too, that other chords can have a 7th added to it too, like supertonic 7th, and subdominant 7th. However the dominant 7th is still more widely used in music, that's why it's important enough to learn early on.