Parentheses

Parentheses can be used to set off supplementary, interjected, explanatory, or illustrative remarks. They are tall punctuation marks "()" used in matched pairs within text, to set apart or interject other text.  

The words placed inside the parentheses are not necessary to understanding or completing the sentence. The words within the parentheses could be removed and a complete sentence would still exists.

Parentheses may also be nested (usually with one set (such as this) inside another set). This is not common in formal writing (though sometimes other brackets [especially square brackets] will be used for one or more inner set of parentheses [in other words, secondary {or even tertiary} phrases can be found within the main parenthetical sentence]).

There are many ways to use parentheses.

Interrupted Sentence

• Jimmy (who we all know is smart) said we should keep searching.
• Be sure to call me (extension 2104) when you get this message.
• Copyright affects how much regulation is enforced (Lessig 2004).
• Sen. John McCain (R., Arizona) ran for president in 2008.

Any punctuation inside parentheses or other brackets is independent of the rest of the text. When several sentences of supplemental material are used in parentheses, the ending punctuation is placed within the parentheses. For example:

• Mrs. Pennyfarthing (What? Yes, that was her name!) was my landlady.

Enumeration 

Parentheses are sometimes used to enclose numbers within a sentence. The purpose of using numbers within parentheses is to highlight multiple points in one sentence. 

• All applicants must submit (1) a cover letter, (2) a resume, (3) a list of references, (4) an essay, and (5) letters of recommendation. 

The numbers within parentheses highlight the items applicants need to include. They are intended to add clarity to the sentence. 

Square Brackets

Square brackets are mainly used to enclose explanatory or missing material, which is usually added by someone other than the original author. This is especially prevalent in quoted text. For example:

"I appreciate it [i.e., the honor], but I must refuse. "

"The future of psionics [i.e., mental powers that affect physical matter] is in doubt."

Modifying Quotations

Square brackets may also be used to modify quotations. For example, if referring to someone's statement "I hate to do laundry," one could write: He "hate[s] to do laundry."

The bracketed expression "[sic]" is used after a quote or reprinted text to indicate the passage appears exactly as in the original source; a bracketed ellipsis "[...]" is often used to indicate deleted material; bracketed comments indicate when original text has been modified for clarity. For example:

• "I'd like to thank [several unimportant people] and my parentals [sic] for their love, tolerance [...] and assistance [emphasis added]."