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).

Parentheses are also used to enclose textual citations and numbers or letters in a list.

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

The in-text citation in this sentence shows the style of the American Psychological Association (APA). (See Chapter 50 for more on APA style.)

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