Many people confuse these two distinct editorial skills, but it's important to recognize how they differ, and why. The most obvious distinction is the form the medium takes. Copyediting, once performed by making marks and writing revisions on a typewritten manuscript, is now generally carried out by entering changes in a word-processing program like Microsoft Word.
Proofreading, by contrast, is done on a facsimile of the finished product — a proof, hence the name. Proofreading is usually still completed on hard copy with a pen or pencil, but it's sometimes accomplished by electronically marking up a PDF (a file created with Adobe's Portable Document Format; that's where the initials come from).
But that's just the beginning. The copy editor's task is to finesse a writer's prose so that it observes all the conventions of good writing. A writer may be skilled at explaining a procedure or verbally depicting a scene, but the copy editor is the one who makes sure the manuscript's syntax is smooth, that the writing adheres to the conventions of grammar, and that wording is proper and precise and punctuation is appropriate and correctly placed.
The copy editor may also do or suggest some reorganizing, recommend changes to chapter titles and subheadings, and call out lapses in logic or sequential slip-ups. This attention is especially important when the content editor — the person who helps the writer shape their prose — has minimal time (or skill) or is absent altogether.
All the while, if the project is a book manuscript, an extensive report, or something else of significant length, the copy editor compiles a style sheet, a statement of overall editorial policy (serial comma, or no? numbers spelled out, or in numeral form?) and a record of idiosyncratic word usage. (Just how do you spell fuggedaboudit? According to the style sheet, just like that — every time.) Many style sheets also list all proper nouns to make sure names are always spelled and capitalized consistently, though search functions and spell-checking programs have rendered that usage somewhat superfluous.
The proofreader, by contrast, is assigned to check a reproduction of what the finished product will look like. And the task is not revision, but correction — making sure that no typographical errors remain from the manuscript or were introduced in the production stage.
New text, such as captions, for example, is often entered separately and may not have been edited. Alternatively, an element — anything from a letter to a paragraph or more — may have been inadvertently omitted or repeated, or misplaced. Because most text is copied and pasted directly from an electronic document, this mishap is unlikely but not unknown.
Then there are esthetic issues: too many end-of-line hyphens in a row, or a word broken in half at the end of a column or page, or a widow (a very short final line of a paragraph at the top of a column).
The proofreader is also the main beneficiary of the style sheet's compilation. Hey, it's fuggedaboutit on page 37, and fuggedaboudit on page 59. Which one's correct? The second spelling, according to the style sheet.
Proofreaders are also expected to check page numbers or recurring copy at the top or bottom of a page that identifies a section in a periodical or a chapter or book title. They make sure the font and type size and weight for one text element matches another element of that class. They double-check that photo captions match the content of the photographs or that when text refers to a table, a chart, or a figure, the graphic element consists of what the text says it does — and they proof that element, too.
Proofreaders may also catch grammatical errors or inconsistency of style, and they are often given some leeway to change or at least call out egregious errors, but they're generally constrained by not being permitted to revise the text in any way that adds or subtracts the number of lines on a page, because doing so may adversely affect the graphic design.
In summary, copyediting is a more qualitative skill and proofreading is more quantitative, though there's quite a bit of overlap, and someone who does well at one often succeeds at the other as well. Proofreading usually pays less and is a pathway to copyediting, but many editors (myself included) do both.
To save time or money or both, many print and online publishers alike have curtailed or abandoned either stage (or, worse, both stages) of the editing process — and it's almost invariably obvious. But there are still enough people out there who value rigorous attention to detail in written expression that the copyediting and proofreading professions aren't going anywhere, and adept practitioners will remain in demand.
Thanks to DailyWritngTips