The original distinction that I'd heard was that a chaconne is based on an
unvarying chord progression whereas a passacaglia was based on an unvarying
melodic line. Usage has been quite inconsistent. I've often heard the Brahms
4th finale referred to as a chaconne even though it's recurring melody line
can always be found (before the code) and the opening chord progression
cannot. Go figure.

Also, a chaconne can be a stately triple-metered dance. A chaconne often
served as the climax for 17th-century French ballet productions.

Aaron J. Rabushka
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http://www.cowtown.net/users/arabushk/