7th chords and modes

Hi. After 6 years of playing guitar, I'm just beginning to learn theory. Recently I learned about the chords that are created when you play up the major scale, and that the order they come in is something like Major, Minor, Minor, Major, Major, Minor, Diminished. And these chords are what determine the key of the song, as well as the modes that sound alright to use with it. So, here's my question: how do 7th chords fit in? I mean, before I knew about this stuff, if I were playing blues in Am, I would just solo in Am, even when the E7 comes around. But in terms of guitar theory, how does that E7 work out?

ok....I take it your referring to Dominant 7th chords right?
Dominant 7th chords are created off of the 5th degree of the major scale, so in the key of C you would have G7.

Now when you move into a minor tonality and you have a Dominant chord appearring off the 5th scale degree, as in the case of E7 vrs A-, you just entered the realm of Harmonic Minor.

7th extensions are different from 7th chords, which are meant as dominant 7 chords. Every chord can have a 7th note added as an extension to its chord structure, either a minor or major depending on what chord it is. If it is the I chord, it is a major 7th - ie, a B in a Cmaj7 chord. on the II and III chords it is a minor 7th. On the IV chord it is a major 7th. On the V chord it is a minor 7th, which makes the chord a dominant 7th chord, as it is the only chord based off the major scale that has a major third and a minor 7th. On the VI and VII chord it is a minor 7. Now, If you are playing a chord like E7, it implies that the E is the V of a chord. If it has a major7th in it is is written like Cmaj7 or CM7, if it is a minor chord it is written Cmin7, or Cm7. In blues, however, you play a I, IV, V progression and all of them are dominant 7 chords. Strange, yes, but blues doesn't fit easily into common theory. Part of what makes the blues sound like blues is the way in which a minor pentatonic scale works over the dominant chords of I, IV, V. In other words, after all of this, you are playing the right thing for blues. If you were to play a scale over a V chord in jazz, you would generally play a mixolydian scale of the same note, ie G mixolydian over G7. That would mean the you could play A mixolydian, D mixolydian and E mixolydian over a I, IV, V blues progression in A, but it wouldn't sound like blues. Give it a go though, it can sound kind of nice.

I think Bingo (cool name-o) is referring to a similar question I posted a while back. There are several examples of chord progressions which are directly derived from the Major scale. The classic example is the II - V - I progression. Using the Cmajor Scale as an example, all of the notes in the II(Dm7), V(G7), and I(CMaj7) chords are directly derived from the C Major scale. There is no need to alter notes from the scale to create the chords. Theoretically, you could play all of the notes from the C Major scale over that progression, even in random nonsensical order, and it wouldn't sound tonally incorrect.

One would think that the I - IV - V Blues progression, because it is so commonly used and versatile, would have a similar derivation -- That the chords for the I - IV - V Blues Progression could also be derived from one common Major Scale without having to alter any tones, but it just doesn't seem to be the case. The I and IV chords have to be altered and at least one note from the Major scale fails to work well over that entire progression. I realize that the Major and Minor Pentatonic scales will always work over the (I7 - IV7 - V7) progression in the same way that the Major scale works over the II - V- I, but I don't believe that the I - IV - V progression was actually derived from those scales. For whatever reason, this commonly used progression seems to go against the rules that work for many other common progressions.