Proofreading is the process of correcting any errors on the proof—that is, the version of the publication that comes after a designer has laid out the text and any images, often using InDesign or another such program. Many proofs come to the proofreader as PDFs and are marked up and edited electronically (although hardcopy is also always an option). Ideally, as little change as possible happens to the text itself at this point.
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Proofreading involves checking and correcting
margins and line spacing
hyphenation across line breaks
cross-references (e.g., footnotes and tables of content)
font sizes and weights
any final style or spelling errors
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Other names for “proofs” include galleys, bluelines, scatter proofs, ... and the list goes on and on.