===Michael - actually I was asking something a bit different, which is how come so many accompaniments seem to use *more* than the three 'obvious' chords.===

Exactly Pip - you make my point for me. A very good question; but one which is obviously going to confuse, rather than enlighten, the OP, who asked a specific question, the simple answer to which your 'asking something a bit different' is going to distract him from. I think your interesting questions would therefore be better put on another thread dedicated to your 'different question' — as a kindness to the OP who wants to know something definite without distraction. Would you not agree?