A while ago I found an electric guitar. I've just been making up my own stuff on it, and I haven't been reading any tab, or playing chords or anything. But I found myself curious of something.
Certain frets sounds 'good' together. I am playing D tuning if that matters, DGCFAD. On the A string, all of these sound good for jamming: 2,3,5,7,9,10,12. These also sound good playing with all of 1,3,5,6,8,10,11. This work similarly with all the other strings, I just found these while playing.
But otherwise, I could play all of these on the A string 2,4,5,7,9,11,12. Etc.
What is the theory that underlies this?