Dominant 7th chords can easily be arrived at if you already know how to play major 7th chords. If you know how to play a Cmaj7 chord, which in its basic form is:
C E G B
then simply taking the 7th of the chord (in this case, the B) and lowering it one half step, which results in:
C E G Bb
When it comes to dominant 7th chords (and other chords), it helps to look at them in terms of their function. Let’s say that we are playing a song in the key of C Major. This means that the melody and harmony are based on the C Major scale (for the most part):
C D E F G A B C
The fifth degree of this scale is G. When it comes to the functionality of harmony, in music we refer to these degrees in Roman numerals:
C is the I
D is the II
E is the III
F is the IV
G is the V
A is the VI
B is the VII
The dominant 7th chord is based on the fifth of the scale that corresponds to the key that we are in. In this case, the fifth or V,
is G. If we build a chord in third intervals starting on G and staying within the key signature of C Major we have:
G B D F
Here we have the dominant 7th chord in the key of C. Because it starts on the fifth (V) of the scale of C Major, this chord is often referred to as “the V chord” (the 5 chord).
So, let’s say that we are playing in the key of F Major. Our F Major scale is:
F G A Bb C D E F (notice we have one flat in this key)
What is the V of this scale or key?
Right, it is the C.
Now, if we build a chord in third intervals starting on this note while staying within the key signature of F Major, we arrive at:
C E G Bb
Here we have the dominant 7th chord in the key of F Major.
The chord symbol for a dominant 7th chord consists simply of the letter name of the chord followed by a “7”…
The first chord above is G7
The second chord above is C7
There is only one dominant 7th chord in any given key. Again, that dominant 7th chord is built on the the fifth degree of the scale that corresponds to the key that we are in. Thus, G7 is the V chord in the key of C Major and C7 is the V chord in the key of F Major.
Explore the dominant 7th chord in other keys and your confidence will begin to soar. You eventually want to know this V chord in all the keys. This is true of all 7th chords as well.
Again, learn a little at a time and become accustomed to actually applying what you are learning. So, if you are mainly familiar with triads and the dominant 7th chord is new to you, learn a couple of them and begin incorporating them into a favorite song of yours while you are still utilizing triads that you know for the other chords. Take this approach and your playing will gradually mature!
PLAY WITH PASSION!