Does proofreading require specialized knowledge?

Yes. Emphatically, yes. This goes far beyond the need for basic writing guidelines. Even a competent editor–one who advises on the structure, flow, characters, and scenes of a manuscript–may not be a competent proofreader. Too many people who are not qualified think they are qualified to serve as proofreaders. They are not.​  You can already tell that I have strong opinions on this question. So, I will explain my position with specifics. For those who want only a passing introduction to what matters, read what’s in bold font.

Grammar, Punctuation, Spelling, Capitalization… and Professional Judgment

Proofreaders must have a very thorough knowledge of grammar rules for the specific dialect in which the book is written. (For example, “proper” British English and American English differ in some cases in spelling, grammar, and punctuation.) There are also grammar rules that are essential to apply, regardless of dialect, such as parallel construction (except in very informal narrations) and use of independent and dependent clauses. Let’s not forget capitalization, which some writers seem to use at random. There is a proofreader’s professional judgment as to when, for example, one might consider adding or removing a comma based on ease of reading rather than on a particular punctuation rule. Finally (well, I’m undoubtedly missing some skills), the proofreader must be constantly on the lookout for ambiguities introduced by the writer’s choice of phrasing.

Style-Guide-Specific Knowledge

Before proofreading a text, the proofreader must orient themselves to apply a specific writing style guide. To do that competently, the proofreader needs to have a thorough understanding of the overall elements of grammar and punctuation. That understanding provides the base on which to understand the specific rules of the chosen style guide. Among the contentious, but easily resolved, punctuation style decisions is the use (or not) of the “Oxford comma.” Sigh. Let’s just go with, whatever you decide, do it consistently. There are also stylistic decisions. For example, many writers and readers now prefer the use of “they” for the third-person singular because that is inclusive of any gender identity.

Common Errors in Everyday Usage

I am currently reading a book that is a memoir— nonfiction, first-person narrator, some dialogue (which must be faithful to what was said, regardless of whether the speaker used “proper English”), and mostly narration (which should use correct grammar, in the case of this memoir). This author is very talented in conveying situations, scenes, and emotions. But she makes some jarring grammar mistakes. For example, she sometimes uses “I” when she should use “me”… and vice versa— that is, she might write “He hated her as much as me.” when she means “He hated her as much as I.”— i.e., “He hated her as much as I did.” not “He hated her as much as he hated me.” (I would, as a proofreader, suggest explicitly using whichever of these restatements is what’s intended so that the reader isn’t wondering.)

Spelling

And then there is the whole matter of spelling. There are self-labeled authors who do not even use a spell-checker before turning a manuscript over to a proofreader. That is unforgivable and, in my mind, means that the person may like to write but they are not committed to their craft to the extent that they must be as a professional. However, even when a writer uses a spell-checker to correct typos, there are many misspellings that spell-checkers do not catch. That is where a proofreader must have a firm command of spelling tens of thousands of words. (Some sources estimate an average active vocabulary for English-speaking adults of 20,000 words and an average passive vocabulary of 40,000 words [How Many Words Does the Average Person Know? – Word Counter]) Beyond their knowledge of how to spell most words, a proofreader must recognize when they are unsure of the spelling of a word and, physically or virtually, reach for a dictionary. But spelling overlaps with basic grammar rules at times— for example, the misuse of “everyday” (ordinary or usual, as in “Our everyday prices are low.”) when the writer means “every day” (daily, as in “We offer low prices every day.”). It is always incorrect to write “We offer low prices everyday!” These terms don’t even have the same meaning!

Attention to Details

Clearly, any competent proofreader must value attention to details and respect for the proper use of language. However, those traits must be applied with a deep and accurate knowledge of the mechanics of writing (grammar, etc.) and of guides to appropriate writing styles.