Proofreader's and Editor's Symbols
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University of Colorado
2002

Abstract:

The following marks are standard proofreading and editing marks. A professional proofreader puts a mark (usually a line or caret) in the line and writes the correction in the margin. An editor makes corrections within the line rather than in the margin (in part because an editor's changes are typically more extensive), which is why editors prefer to work with double-spaced copy.