[starsailor]

Interesting question, I suppose if you play the original Ad lib of a song that you have heard someone else do, then you'ld already know the song in that form so in theory you'ld neither be ad-libbing or improvising as you wouldn't be making it up, if you wanted to put your own stamp on the song, you could make some extra words or do it in a different style, then you'd be improvising, actually found another word for these Extemporize which is cool. Hope this makes sense.