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In her work, The Writer Got Screwed (but didn't have to), Brooke A. Wharton debunks a few myths surrounding copyright. You might be well served to pick up a copy of the book for the copyright related content alone. One thing she explains is that a writer has copyright in a work the instant an idea is put on paper (or saved in a computer file). That said, she still advises registration, so you may sue for compensation, if necessary.