Use parentheses to enclose material that is of minor or secondary importance in a sentence—material that supplements, clarifies, comments on, or illustrates what precedes or follows it.

Inventors and men of genius have almost always been regarded as fools at the beginning (and very often at the end) of their careers.

– Fyodor Dostoyevsky

During my research, I found problems with the flat-rate income tax (a single-rate tax with no deductions).

Enclosing textual citations

Freud and his followers have had a most significant impact on the ways abnormal functioning is understood and treated (Joseph, 1991).

– Ronald J. Comer, Abnormal Psychology

Zamora notes that Kahlo referred to her first self-portrait, given to a close friend, as “your Botticelli” (110).

The first in-text citation above shows the style of the American Psychological Association (APA); the second, the style of the Modern Language Association (MLA).

Enclosing numbers or letters in a list

Five distinct styles can be distinguished: (1) Old New England, (2) Deep South, (3) Middle American, (4) Wild West, and (5) Far West or Californian.

– Alison Lurie, The Language of Clothes

With other marks of punctuation

A period may be placed either inside or outside a closing parenthesis, depending on whether the parenthetical text is part of a larger sentence. A comma, if needed, is always placed outside a closing parenthesis (and never before an opening one).

Gene Tunney’s single defeat in an eleven-year career was to a flamboyant and dangerous fighter named Harry Greb (“The Human Windmill”), who seems to have been, judging from boxing literature, the dirtiest fighter in history.

– Joyce Carol Oates, “On Boxing”

Choosing among parentheses, commas, and dashes

In general, use commas when the material to be set off is least interruptive (39c, e, and f), parentheses when it is more interruptive, and dashes when it is the most interruptive (44c).