Up until recently, many professional proofreaders worked in Microsoft Word. Therefore, they rely on their attention to detail and knowledge of English grammar. They also regularly refer to the dictionary, thesaurus, and writing style books. However, professional proofreaders rely on premium software like Grammarly and other tools. While proofreading, they usually change the formatting of an article to Courier and one and a half or two times spacing and scan it for errors and mistakes manually. Sometimes, they print out a piece of writing, check for issues and mark it up with a red pen.