What is the difference between editing, copy-editing and proofreading?
They are distinct stages performed in a fixed order, and conflating them is the most common reason a manuscript gets worse rather than better.
Developmental editing (also structural or substantive) works on the whole. Is the argument coherent? Is the structure right? Is anything missing, redundant or in the wrong order? Is it pitched at the right reader? A developmental editor may recommend cutting a third of the text, reordering chapters, or rewriting the opening entirely. It happens first, because there is no point polishing sentences that will be deleted.
Line editing works at paragraph and sentence level on how it reads. Rhythm, clarity, transitions, tone, repetition, overlong sentences, weak verbs. It is concerned with quality of expression rather than correctness, and is where most of the improvement in readability comes from.
Copy-editing works on correctness and consistency. Grammar, punctuation, spelling, agreement, and adherence to a style guide — whether you write per cent or percent, 1990s or 1990's, Oxford comma or not. A copy-editor also queries factual inconsistencies, broken cross-references, and anything that contradicts something forty pages earlier. Copy-editors typically maintain a style sheet recording every decision.
Proofreading is the final check on the typeset or formatted version. It catches typographical errors, bad line breaks, wrong page numbers, misplaced captions, formatting inconsistencies and anything introduced during layout. It is not the stage for rewriting, and requests to reword at proof stage are expensive and risky.
Why order matters: each stage is more granular and less reversible than the last. Proofreading a draft that will be restructured wastes the work entirely.
Practical advice for your own writing: separate the passes rather than doing everything at once. Read aloud for line editing, and read backwards, sentence by sentence, for proofreading — it stops you reading what you meant to write.