Parentheses ( )

How to read it

The American Heritage Dictionary defines them as the upright curved lines used to mark off explanatory or qualifying remarks, or to enclose a mathematical expression treated as a whole. The Oxford Advanced Learner’s Dictionary labels parenthesis formal or North American, with bracket and round bracket as the British English words, and the American Heritage Dictionary marks bracket in this sense “chiefly British”. Unicode encodes them as U+0028 LEFT PARENTHESIS and U+0029 RIGHT PARENTHESIS, read as opening and closing rather than by shape, so in right-to-left text the same character is drawn facing the other way. Butterick’s Practical Typography sorts the family: parentheses for citations and other asides, [square] brackets for changes within quoted material, and {braces}, or curly brackets, mainly for technical and mathematical writing. The US Government Publishing Office’s Style Manual uses parentheses for references such as (see fig. 2), for a clause too great an interruption for commas, and round the letters or numbers of items in a series: (a), (b), (c).

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Brackets or parentheses?

The names change across the Atlantic. In American usage, as the American Heritage Dictionary and Butterick use them, parentheses are ( ) and brackets are [ ]. In British usage, according to the Oxford dictionary, ( ) are brackets or round brackets and [ ] square brackets. The Guardian’s style guide files the rule under both words, and uses square brackets for an editor’s note inside a direct quotation, as does GOV.UK.

Punctuation inside or outside

The Guardian’s rule: if the sentence is complete without the words in parentheses, its punctuation stays outside them (like this). A complete sentence that stands alone in parentheses starts with a capital letter and ends with its own full stop inside. The GPO Style Manual sets out the same placement of the period.

Author(s): the plural in brackets

Chicago and the UK government disagree here too. The Chicago Manual of Style’s Q&A accepts parentheses for an optional plural ending, as in “return your manuscript to the author(s)”, though it finds forms such as warranty(ies) start to become unclear. GOV.UK’s style guide rules it out: “Do not use round brackets to refer to something that could either be singular or plural”, and writes the plural instead, documents rather than document(s).

In maths

The American Heritage Dictionary’s definition includes enclosing a sum or product treated as a collective entity, and Oxford’s example is “Add the numbers in brackets first.” Unicode notes that in some mathematical usage brackets are deliberately left unpaired or reversed, as in ]a,b[.

Look-alikes

Square brackets [ ] (U+005B, U+005D), braces { } (U+007B, U+007D) and angle brackets are separate characters with separate jobs (Butterick, Unicode). The Slash (/) is the other common way to write an alternative, as in on/off.