Building Dominant Chords
You may wish to review Building Major & Minor Chords before proceeding with this lesson
Dominant 7 chords are essentially a Major triad with a minor 7th added as extended harmony.
Once you've established a major triad, there are a couple ways to figure the 7th; you could either find the seventh a step and a half above the fifth or a whole step below the root (the root being also the eighth).
Here are a few examples:
G Major: root - G, third - B, fifth - D
A step and a half up from D (fifth) is F. Likewise, a whole step down from G is F
So G7 is spelled G, B, D, F
C Major: root - C, third - E, fifth - G
A step and a half up from G (fifth) is Bb. Likewise, a whole step down from C is Bb
So C7 is spelled C, E, G, Bb*
*The seventh must be called Bb and not A# (which is enharmonic or exactly the same pitch) because A# would be a sixth
F Major: root - F, third - A, fifth - C
A step and a half up from C (fifth) is Eb. Likewise, a whole step down from F is Eb
So F7 is spelled F, A, C, Eb*
*Again, the seventh must be called Eb and not D# (which is enharmonic or exactly the same pitch) because D# would be a sixth