A student from many years ago recently told me "The most useful thing I ever learned from you was how to figure out those weird chords that often show up in a song."
It was eye opening to me also when in my first college level theory class, after the chapters on Primary Chords & Secondary chords (all of which I'd been playing but not understanding why), there came this chapter nearly half way through the text titled Secondary Dominants They included many of those chords we all use beyond the only seven that can exist in any key. You know the chords that can in no way belong to the key in which one is playing?
Way down the page explaining this concept of classical theory was this little phrase .... "Some theorists refer to these as Borrowed Chords" and the light bulb went on in my head! I thought, "That's something any player can understand!" I've expanded that idea to include the six most commonly occurring odd chords encountered in most genres, Jazz being a different animal altogether. I use the concept in all my beyond beginning instruction & books ever since. I'll discuss Jazz in an upcoming post.
The attached chart might be a way to organize your thinking around all .... those other chords?Attachment 180747