Susan-Marie & Jon beat me to it - when I'm trying to work out accompaniment for a song, I try to "ear" the last note of the song, use that note for the root I chord, and try it from there. If the song ends on a note that sounds like "A", I start with an "A" chord and see if I can pick the melody in the "A" position. Usually I can get several melody notes, then I miss a note, so I'll try the notes of the IV (D) or the V (E). Occasionally I have to use the IIminor. Most of the time it works - of course, I usually am concerned with folk, bluegrass or Irish songs. But after all, they're the ones I really love. To follow up on Bill D's comment about the layout of a 21 bar autorharp - I don't know anything about autoharp, but a good way to learn folk keys and progressions is the hammered dulcimer - its notes are arranged in kind of a "circle of changes" where the keys progress right through each other. Woody Guthrie once said he tried to only use 2 chords, that anything more was just showing off. He did admit to throwing in an extra chord once in a while, just to impress the ladies in the crowd... Then there are those horn players; I pretend to know nothing about flatted scales. String players are sharp!

Country Bob