A “dominant 7th chord” is constructed in the same way as a major 7th chord with one

important difference.  Instead of adding the 7th note of the major scale to our

3 note major chord (C,E,G), we add the note one semitone lower. 

On a piano this means one key to the left. 

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The 7th note in the key of C major is (as we saw previously) the note B.  The note one semitone lower

than B is the note Bb.  This is the black key on a piano between the white A and B keys.

 

This note is called the flat 7th (b7) of the C major scale.

 

.  So a C dominant 7th (written C7) can be played using these 4 notes on a piano.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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                   Playing the same 4 notes as a chord on a guitar in the

               same order is awkward, but the order is not too important.

                    We need to keep the root note C as the bass (lowest

                 sounding) note, but can put the others wherever we like.
                               Here's a C7 chord barred at the 3rd fret.

 

 

 

 

 

 

 

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                                                                           And here’s the most common fingering

                                                                                                       for a C7 chord on guitar.                            

                   

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Note that in the fingering above we have omitted the note G from our 4 note 7th chord.  This is an example of changing a chord to suit the complexities of fingering on a guitar.  Strictly this chord should be called C7 (no 5th).

 

 Despite omitting the G the chord retains the tense “bluesy” sound of the dominant 7th.  It's the 3rd and the flat 7th (E and Bb) played together which create that sound, so the 5th (G) isn’t missed.

 

 

To create more complex dominant chords, we can continue as before, but each time using the

flat 7th note (Bb) rather than the major 7th note (B) in our chords.

This is the only difference between a maj7 and a (dominant) 7th chord, a maj9 and a (dominant) 9th chord etc.

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So...

C = 1,3,5 = C,E,G
C7 = 1,3,5,b7 = C,E,G,Bb
C9 = 1,3,5,b7,9 = C,E,G,Bb,D
C11  = 1,3,5,b7,9,11 = C,E,G,Bb,D,F
C13 = 1,3,5,b7,9,11,13 = C,E,G,Bb,D,F,A

 

 

 

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And that takes care of the foundations on the understanding side of things.

There really isn't that much to it.  The real work as
ever on a guitar is getting to be able to use this information, ie. incorporating
the new knowledge into your playing.