To form a seventh chord, you take a Major or Minor Chord and add
another note on top. However, there are several different kinds
of the Seventh Chord which you need to be aware of. Example:
This is the scale of G again: The seventh note of the scale is
an F# and adding this to G Major would give a G Major Seventh
chord. If you flatten the seventh note to F you get an
'ordinary' seventh (called a dominant seventh - G7). If you
flatten the 3rd (to make a Minor Chord) and flatten the 7th,
you'd have a G Minor Seventh Chord. Take a look at the table
below: |G major seventh||G||B||D||F#| |G dominant
seventh||G||B||D||F| |G minor seventh||G||Bb||D||F|