Four note chords are called
7th chords because they are made up of the Root, 3rd, 5th and 7th notes
of a scale.
However, the first use of a four note chord was a chord built on the
5th degree of the scale. (Dominant)
That is why they are called V7 or Dominant 7th chords.
This also means that we don't use the Key
Signature of the Root note of the Chord, but the Key Signature of the
scale which has the Chord Root as its 5th degree.
Ie. A G7 chord does not come from a G major scale ; It comes from the
scale where G is the 5th degree; the Key of C.
Or you can simply build the four note chord, use the Key Signature of
the Root note but then lower the 7th. |