Parentheses are used around numbers showing dates (usually dates of birth and/or death), inserted figures, or numbers or letters in an itemized series (such as a series of steps).
Date: Joshua Chamberlain (1829-1914) received the Congressional Medal of Honor for his role at Gettysburg.
Inserted figures: The Senate vote was very close (50-48).
Numbers in a series: To make New England Clam Chowder get the following items: (1) quahogs, (2) cream, (3) potatoes, and (4) onions.
Letters in a series: What was Hamlet's mother's name?
(a) Ophelia (b) Beatrice (c) Gertrude (d) Helena
A single closing parenthesis follows the number or letter in specific divisions of a formal outline.
Example:I. GrammarA. Punctuation1. Parenthesesa. Parentheses with interrupting elements
b. Parentheses with certain numbers or letters1) Dates
2) Inserted figures
3) Itemized seriesa) Numbers
(Some authorities enclose the numbers and letters in parentheses in an outline like this as with the letters and numbers in a series above.)