How do the two work together? I've been told that generally when you write a song, to go about it in the traditional fashion, would be to select a key first, and then the key determines which chords are available, and the melody comes from the scale tones of the given key. How does one know how to use a specific chord progression to support a specfic melody? or vice versa? What I mean in nutshell is, how does harmony function in terms of supporting a melody? Are there any basic rules that apply, or one should abide by?