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Parentheses enclose material of minor or secondary importance in a sentence—material that supplements, clarifies, comments on, or illustrates what precedes or follows it. Parentheses also enclose numbers or letters that precede items in a list, and sometimes they enclose source citations or publication information.

Parentheses for less important material

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).

A period may be placed either inside or outside a closing parenthesis. If the parenthetical material is part of a larger sentence, put the period after the parentheses; if the entire sentence is in parentheses, put the period inside the parentheses. 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”

If the material in parentheses is a question or an exclamation, use a question mark or exclamation point inside the closing parenthesis.

Our laughing (so deep was the pleasure!) became screaming.

—RICHARD RODRIGUEZ, “Aria: A Memoir of a Bilingual Childhood”

In general, parentheses create more of an interruption than commas but less of an interruption than dashes.

Parentheses for 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

Parentheses for textual citations

The first of the following in-text citations shows the style of the American Psychological Association; the second shows the style of the Modern Language Association.