in Editing
What a copy-editor actually checks
Copy-editing is not a quick tidy of the commas. It is where a book is made consistent, and consistency is harder than it sounds.
Copy-editing sits between line editing and proofreading, and it is the stage writers most often underestimate. It is sometimes described as "checking the grammar", which is true in the way that a surveyor "checks the walls".
Correctness
The first layer is the one people expect. The copy-editor corrects spelling, grammar, punctuation and syntax. They catch the dangling modifier, the subject that no longer agrees with its verb after a late revision, the missing closing quotation mark, the "its" that should be "it's". They also flag sentences that are grammatical but ambiguous, where a reader could take two meanings and the writer clearly intended one.
This layer is necessary but not the heart of the job. Spellcheckers catch a share of it. What they cannot do is the next part.
Consistency
A copy-editor keeps a style sheet as they work: a running record of every choice the book makes. Is it "grey" or "gray", "OK" or "okay", "towards" or "toward"? Is the house on "Mill Lane" or "Mill Road"? Does the character spell her name "Katherine" in chapter two and "Catherine" in chapter nineteen? Are numbers written as words or figures? Are thoughts in italics or not?
None of these choices is wrong in itself. What is wrong is making them differently on different pages. Readers may not be able to say what bothered them, but inconsistency reads as carelessness, and it erodes trust in the rest of the text.
The copy-editor also tracks continuity: eye colours, ages, the day of the week, who was in the room, whether the coffee is still hot three pages after it was poured. In non-fiction, they check that cross-references point to the right chapter, that headings follow a consistent hierarchy and that the notes match their numbers.
Sense and fact
A good copy-editor queries anything that looks doubtful. A date that does not fit the historical period. A street that does not run where the text says it does. A quotation attributed to the wrong person. A calculation that does not add up. They do not usually rewrite to fix these; they raise a query and leave the decision to the writer.
Copy-editors also watch for material that could cause legal trouble, such as a real person described in a damaging way, or song lyrics quoted at length. They are not lawyers, and a flag is not a legal opinion, but noticing is part of the job.
What they do not do
A copy-editor should not restructure the book or rewrite the voice. If a chapter is in the wrong place, that is a structural matter that should have been settled earlier. If the prose style is deliberately unconventional, the copy-editor's task is to make it consistently unconventional, not to normalise it.
This is why it matters that the manuscript is stable before copy-editing begins. Every substantial rewrite afterwards introduces new inconsistencies and undoes checked work.
When the copy-edit comes back, expect many small marks and a set of queries. Answer every query, even if the answer is "keep as is". The style sheet that comes with it is worth keeping too; it will save time at every later stage.