In the world of fiction writing, copyediting includes critical points like proofreading for spelling and punctuation, grammar-and-style considerations, formatting, continuity, and lots of other things that will shoot you in the foot as surely as Tuesday comes after Monday if not taken care of before you submit your manuscript.
Proofers and copy editors are not developmental editors, also called content editors or book doctors. Unlike them, a good proofer and copy editor will always improve your work because structural revisions will always make your work linguistically stronger, and because any revision suggestions from The Proofing Department are supported by not only twenty-one years of copyediting experience but also by all top-shelf references such as the Chicago Manual of Style and Merriam-Webster's Unabridged Dictionary. Professional proofing and copyediting prior to submission of your manuscript is the single most beneficial thing you can do for your final draft; in fact, the issues covered by the proofing process cumulatively comprise the single biggest reason manuscripts are rejected within the first five pages.
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