John Coles is investigating the difference between throws and takedowns in the martial arts for his forthcoming book, and has written a couple of blog posts on the subject. John points to some discussion of the various takes on the difference this thread from the Martial Arts Planet forum.
An empirical exploration that anyone (with a bit of experience) can do is go through their style's syllabus and pull out the techniques explicitly labelled as throws and those labelled as takedowns and look for any differences of principle.
That's what I intend to do!
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