Amish Guitar

Lately I've been working on finding interesting (to me) instead of specific songs. Unfortunately I don't know much beyond the basic I IV V variations. One progression that I keep coming back to is:

G D C
G D C
G D Em Am
Em C
D

Am7 Em
Am C
D

G D C
G D C
G D C
D
G

I vary the rhythm to keep it interesting.

Another one I do is:

C D
G Em
C D
G
C D
G Em
C D7
G

Now, that one is quite familiar but I can't quite figure out what song(s) use the pattern.

The chord progression from Roy Orbison's Cryin' is kinda fun

D D+ G Gm
D A7 D

Well, that's just a short snippet but it was kinda fun to learn.

Another one I'm having fun with is:

D A Bm F#m G D G A

Which also happens to be the progression for Pachelbell's Canon in D. I found this one online however. It came up in a songwriting lesson on about.com that compared Pachelbell's masterpiece with Green Day's Basketcase. Interesting. The same progression for both (I V vi iii IV I IV V), just different keys. It does tell me that songs written in D would have the B and F# chords as minor, and I'm pretty sure the II chord (E) would be minor as well. And the seventh tone is almost always a form of a 7th chord (coincidental?) so VII would be C7.

So,what have I learned with all this mumbo jumbo. Just this, if I were to write a chord in a major key, the triads would be I ii iii IV V vi VII7. Good to know.

Now, can someone tell me when to add 9?