Sevenths

E7 is the V chord of A major or A minor

Eminor is the relative minor of G major - but it's not commonly turned into E7. It's much more commonly seen as E minor 7th. If it appears as E7 there's a good chance that it's about to change key to A or Am

In old swing tunes a dominant on VI is quite common. So in the case of E7, you'll find it often in swing tunes in the key of G (it is called a secondary dominant since it is not THE dominant of G).

Some examples:

"Sweet Georgia Brown" (in the key of G)

E7 A7 D7 G

"All of Me" (in the key of G)

G B7 E7 Am

A lot of secondary dominants as you see...very easy to improvise over :)

Depends on the music.

If you stay with diatonic harmony, it's in A or A minor (using the harmonic minor scale) as Fretsource noted.

If you follow secondary dominants, you can be in any key - but it will lead to an A type chord - as in Niliov's examples.

If you're playing blues, it can be in the key of A, B, or E - it's common to use dominant chord types on the I, IV, and V. Sometimes those dominants function like they 'should' (E7->A7), sometimes they don't (E7->F#7 or E7-B7). That's the blues for ya :)

Often in blues you'll switch from a regular major triad to a seventh while you're staying on the I for a while. A seventh can go in place of the major triad anywhere in the I, IV, V.

This topic is interesting:

Based on Noteboat comments, I would like to know why Blues music allows a dominant in I or IV.I mean what is the theory behind it?

The short answer is "because it sounds good". Really.

Music doesn't use a particular device because theory says it can - it's the other way around. Theory tries to explain what's being used in music.

In classical music, theorists have had a few centuries to think about the music and create the "rules" of theory. Blues hasn't been around that long - only about 100 years according to many musicologists. So it's light on explanations.

But I'll try :)

In major/minor tonality, the V7is a natural extension of the scale in triads. Dominant sevenths contain a major triad, which occurs on only three scale degrees: I, IV, and V in the major. Extending the next third on each of these tones results in only one dominant seventh; the other two create major sevenths.

If you look at the tonic's scale degrees against the chords, you find:

I is 1-3-5-7
IV is 4-6-1-3
V is 5-7-2-4

3 and 7 are major compared to the roots of IV and I; 4 is flat compared to the scale of V.

But look at the blues scale - it flats both 3 and 7 - the exact tones that would make I and IV major sevenths in a major scale. So... dominant chords are the natural extension of all major triads using the blues scale.

Excellent post, Noteboat.
Nuff said!!

Something I noticed is that quite often a given minor chord is made major if it is a fitfh above or a fourth below the next chord. For example, the chord progression of New York State of Mind. It's in C, and normally you'd expect Cmaj7-Em9b-Am-G, but the Em9b is made E9b as that leads well to the Am chord.

In that situation you've essentially created a secondary dominant, even though you go right back to G.

E major is the dominant of both A and Am. Dominant chords like to "lead" or "pull" to the tonic chord.

By changing Em to E preceding an Am, you change the third of the chord, G, to G#, creating a "leading tone" (or the seventh scale degree) of A. Leading tones (the 7th degree of your scale or the third of the dominant chord) want to resolve. E creates more pull to A than Em because of that G#. Adding a seventh to the chord creates even more pull, because that seventh wants to resolve a half step down (the opposite direction of the leading tone). A dominant seventh chord also wants to go somewhere with more urgency than a normal dominant because the third and seventh of the chord form a "dissonant" tritone or augmented fourth interval between them (I quoted that because it isn't a normal sort of ugly dissonance, but it is considered dissonant in theory. Back in earlier centuries, every interval but the 3rd, 5th, 6th, and octave was considered dissonant, and the augmented fourth was even considered the "devil in music" :twisted: ) So in short, it isn't always just the interval between the chords that determines what you do with it.

One of my favorite movies is Amadeus, and they really use that musical structure to create tension throughout the movie. They choose fantastic portions of music which come to these monumental dominant 5 chords, and then just cut them off abruptly. It leaves you hanging. It's devious, but very intelligent use of music by the filmmakers.