Proofreaders
United States
The United States has a large and highly specialized proofreading market, with many professionals focusing on a single genre or manuscript type, from commercial fiction to academic and business nonfiction. American proofreaders typically work to Chicago Manual of Style conventions, the standard reference for most US trade publishing.
Site list
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Conclusion
Ask which style guide a proofreader typically works from, and confirm they're comfortable adapting to a different one if your project calls for it, since conventions can vary by genre and publisher.