With the thread focused on the Dominant note, this would be a good time to introduce Secondary Dominants ( CSchlegel will probably add some great insight to this topic, if we're lucky). His post introduced Roman Numerals and what they stood for, ie. V= Fifth and vi= sixth ( Capitols= Major and lowercase= minor). With that said, each note in a scale has it's own dominant note, for instance the C Major scale is C=I, d=ii, e=iii, F=IV, G=V, a=vi and b=vii. An example of a secondary dominant would be written as
"V of V" or V/V, which means the "fifth of the fifth". The fifth of C is G and the fifth of G is D, so "V of V" would be D. Again from the same C Major scale
"V of ii" or V/ii would be A because A is the fifth of D. etc......
If you're wondering what this has to do with anything? The answer is a lot.
The use of fifths does not have to be confined to the immediate key of a song. Take the song "Bell Bottom Blues" by Clapton: Chords are C-E7/B-Am.
The E7 chord uses G# which is not in the C Major scale. Also E is the iii, so it should be assumed as an Em chord. The trick is not to think of it as the iii but the V of vi (E is the fifth of A), creating an E dominant 7th chord. Using the dominant of a note is usually played in Major form.