If you're trying to learn a song by ear for example, how do you find the backing note to go along whith what you are playing?
For example, when playing canon in D, you first start with the D string then progress down, next you pluck the A string and do the same, etc...
So, I wanted to know how do you find it out? What are the "rules" or something like that? Like scales? But how do I know which?
Also, I have played songs when the same note may have different backing notes...
EDIT: as it was commented, I believe what I meant was Harmony, that is what I would like to know.
EDIT2: I want to know how do I find the bass note on a song, I know that in Pachelbel's the bass note is usually the root note of the chord, but I have also played some other notes where the bass not is different from the other note you are playing at the same time