Most music has a structure called a chord progression, that is, chords within the key that follow a certain pattern. The most widely used is called a I-IV-V (1,4,5) progression. For example, in th key of 'C':
C-I (root or tonic)
D-II
E-III
F-IV
G-V
A-VI
B-VII
C-VIII/I (root one octave higher
by counting off, F and G are the IV an V respectively. In the I-IV-V progression, the chords are C, F, and G. G is the last chord played and finishes or resolves the sequence and the progression will start over or end on the root chord (C in this case).
There are many different Chords out there, but called the "7" is most commonly used on the V chord of a progression. It is within the key signature of the key, and gives an extra bit of urgency in resolving the progression. Blues often use sevenths for the I, IV, and V, giving the blues their characteristic, edgier sound. slapping sevenths in all three chords goes outside the key signature and gives a more meloncholy undertone. Without going into every key, the seventh for F is E flat, which is the minor 3rd for C, the note that makes the chord a minor chord, or a "sad" chord. Normally this is E natural and makes the chord sound "happy". In addition, solos usually use the minor pentatonic (5 note scales) on top of this progression to further give a darker sound.


