Quick answer: Use parentheses ( ) to tuck a nonessential aside — an afterthought, a clarification, an example — into a sentence, where it could be lifted out without breaking the grammar. Use square brackets [ ] to mark words you have inserted into someone else's quotation. End punctuation normally sits outside the closing parenthesis; it only moves inside when a complete, standalone sentence lives within the marks.
Parentheses: for your own asides
Parentheses (the curved marks, called round brackets in British English) hold extra information the reader can do without. The gold-standard test is simple: if you delete the bracketed material and the sentence still reads correctly, parentheses are the right tool.
- ✓ The museum (founded in 1913) reopens on Monday.
- ✓ She promised to call back (she never did).
- ✓ Add two cups of flour (plain, not self-raising) and stir.
Remove the aside from any of these and a complete sentence remains — that is exactly the point.
Punctuation around parentheses
This is where most mistakes happen, so here are the rules that actually matter.
Never put a comma directly before an opening parenthesis. If the sentence needs a comma there, it waits until after the closing parenthesis.
- ✗ Although he apologized, (twice) she stayed angry.
- ✓ Although he apologized (twice), she stayed angry.
Keep end punctuation outside when the parenthetical is part of a larger sentence.
- ✗ We finally left the party (around midnight.)
- ✓ We finally left the party (around midnight).
Put the full stop inside only when the parentheses enclose a whole standalone sentence of their own.
- ✓ We finally left the party. (It was well past midnight.)
- ✗ We finally left the party. (It was well past midnight).
Question marks and exclamation marks go inside if they belong to the aside, outside if they belong to the main sentence.
- ✓ Her new book (have you read it?) is about grief.
- ✓ Have you read her new book (the one about grief)?
Capital letters inside parentheses
An aside embedded in a sentence starts with a lowercase letter and takes no full stop of its own. A parenthetical that stands as its own sentence begins with a capital and closes with its terminal punctuation inside.
- ✓ He passed the exam (barely) and moved on.
- ✓ He passed the exam. (Barely.)
Square brackets: for editorial insertions
Square brackets do a different job. They signal that someone other than the original speaker or writer has stepped into a quotation to add, change, or clarify something. If parentheses are the author's own voice, brackets are the editor's.
To add clarifying information that wasn't in the original:
- ✓ The witness said, "He [the defendant] never left the room."
To change a letter's case so a quotation fits smoothly into your own sentence:
- Original: The results were inconclusive.
- ✓ The report concedes that "[t]he results were inconclusive."
To flag an error with [sic] — Latin for "thus" — showing the mistake was in the original and is not yours:
- ✓ The banner read "No tresspassing [sic]."
Brackets also mark an omission you have made inside a quotation, usually written as [...], and they can hold a translation or a short editorial note. In every case the message is the same: this material was not part of the original wording.
Nesting: brackets inside parentheses
When you need a parenthetical inside another parenthetical, don't stack two round brackets. In American usage, put square brackets inside the parentheses.
- ✗ The findings (see the appendix (pages 50–60)) were clear.
- ✓ The findings (see the appendix [pages 50–60]) were clear.
Better still, rewrite to avoid nesting altogether — deeply nested brackets are hard to read.
Parentheses, commas, or dashes?
All three can set off nonessential information, but they differ in volume. Commas are neutral and smooth. Dashes are loud — they spotlight the aside. Parentheses are the quietest, almost a whisper, tucking the aside out of the way.
- My brother, a chef, cooked dinner. (neutral)
- My brother — a chef, no less — cooked dinner. (emphatic)
- My brother (a chef) cooked dinner. (understated)
Choosing among them is a matter of tone, not correctness.
Tricky cases and common myths
"Parentheses don't belong in formal writing" is a myth. Academic and professional prose uses them constantly — for citations, such as (Smith, 2020); for defining an acronym on first mention; and for numbering items (1), (2), (3). The real caution is overuse: if a sentence needs three sets of brackets, rewrite it.
Defining acronyms: introduce the full term first, put the short form in parentheses, then use the short form from then on.
- ✓ The National Health Service (NHS) is free at the point of use. The NHS employs over a million people.
The optional-plural "(s)" — as in applicant(s) or child(ren) — is fine on forms and in instructions but reads awkwardly in flowing prose, where "one or more applicants" is usually cleaner.
One mark or two? A "parenthesis" (singular) is one curved mark or the inserted phrase itself; "parentheses" (plural) are the pair. And always close what you open — a lone opening bracket with no partner is a classic typo.
A memory trick
Two quick tests keep you right. First, the removal test: if the sentence survives without the bracketed words, parentheses fit. Second, for choosing the shape: parentheses are for the author, square brackets are for the editor. Your own aside gets round marks; a change to someone else's words gets square ones.
US vs UK: a real terminology trap
The biggest transatlantic difference isn't a rule — it's a name. In American English, ( ) are parentheses and [ ] are brackets. In British English, ( ) are usually just brackets (or round brackets), and [ ] are square brackets. So when a British editor says "put it in brackets," they mean the curved ones; when an American says the same thing, they may mean the square ones. When there's any doubt, say "round" or "square" and no one can misread you. The punctuation rules above are identical on both sides of the Atlantic.
