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Beginner's Guide to Adobe Flash/Symbols/Movie Clip
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Saving the best for last, the Movie Clip has it all, Movie Clip Symbols are basically movies within the main movie. Movie Clips have an independent timeline in that their timeline is not dependent on the main timeline; therefore their play head can move in sync or out of sync with the main play head. Also, Movie Clips can have ActionScript within their own timeline and/or have ActionScript in their object actions. Movie Clips can contain sounds, actions, and other symbols. A Movie Clip can also be placed inside other symbols such as for animated buttons.