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?