"DOMINANT SEVENTH CHORD"
its defined in the manual as.... "any dominant seventh chord is some Key's V (Five) Chord."
and hell, it doesnt make sense at all to me... 
I'll try to explain, given my very limited understanding of music theory.
First of all, what is a V (five) chord? It's the chord built from the fifth degree (note) of a scale.
In the case of the C major scale (C-D-E-F-G-A-B), the fifth degree is G (C-D-E-F-
G-A-B).
Second, how do you build a 7th chord from the fifth degree of C major? Chords are built on
thirds. Each note in a chord is a third (major or minor third) away from the previous note.
So to build the chord you start with the 5th degree, then go up the scale in thirds until you accumulate 4 notes:
C-D-E-F-
G-A-
B-C-
D-E-
F-G-A-B.
Ergo, the C major scale's V chord consists of the 4 notes: G-B-D-F. This particular combination of intervals (root, major third, fifth, flat seventh) forms a dominant seventh chord. In this case, since the root of the chord is G, it's a G dominant seventh, or more commonly G7.
This particular combination of intervals (root, major third, fifth, flat seventh) is also unique to the 5th degree of scale. If you were to construct chords starting with any other degree aside from the 5th, you would end up with either a major seventh, minor seventh, or half diminished chord. Only the V chord is a dominant 7th.
Now, everything I just wrote above is just a roundabout way of saying that
- G7 is the V chord of C major
- C7 is the V chord of F major
- F7 is the V chord of Bb major
- Bb7 is the V chord of Eb major
(and on and on around the circle of fourths...)
The sound of this chord (G-B-D-F) is very distinctive. It sounds like it wants to resolve to the CMaj7 chord (C-E-G-B). Try playing G7-CMaj7 a few times and you'll hear what I mean.
I hope this clarifies more than it confuses.
.png "lol")
(Feel free to correct me or clarify what I just wrote above.)
