Expanding chord progression repertoire when composing?
I'm working on writing some sad sounding songs. Every time I'm jamming by myself on the guitar, I can play a few chords and sing or whistle what the leads could be. Leads come to me easily, and I can create vocal melodies and guitar melodies that seem pretty interesting to me. But my rhythm guitar chord progressions invariably end up as some combination of Em, C, D (sometimes Em, C, G, D) or Am, F, G. So this sounds pretty sad, and it's easy to write some leads over it, and the leads definitely make things more interesting, but I need to be more creative. Every song can't have that same chord progression. (or a transposition of it) I've tried adding in more chords, but it doesn't sound right. And I've tried doing key changes too, it also doesn't sound right. While the key changes can make things sound "evil" it doesn't make it sound sad, in fact the sadness seems to be lessened by adding in extra chords. One idea I have is trying sus chords, as those can definitely add a nice flavor and can still fit in with the theme of a sad song. I also like building up several major chords before going back to a minor chords as the root, as it sort of has the effect of building up "false hope" in the listener before crashing back down to a minor sound...basically I think why Em C D works so well. And of course I can play around with variation of the chords, like arpeggios, extra notes, etc, but that doesn't change the basic chord progression for the rhythm guitar, and it's getting into lead guitar territory anyway (ie avoiding the problem I am having rather than trying to solve it) And it's also easy to play around with patterns, timing, etc, etc, but again that doesn't get around the fact that the rhythm guitar is still playing the same three damn chords. Voicing is two guitars and vocals. Happy to keep experimenting but what exactly should I investigate now? What aspect of music theory should I study? I don't know anything about modes, is that an area worth study and perhaps is applicable here? Or perhaps there are new chord shapes I can look at? (I'm really just doing major and minor chords at the moment) Or what else would you recommend? tl;dr: How can I keep the rhythm guitar parts sounding interesting but still keep the emotional content that I want? (And what aspects of music theory should I study and what books could I look at?) Thanks!