Use parentheses () in pairs to provide supplementary context (such as examples, to add an aside, or to introduce an abbreviation). Place file extensions inside parentheses after writing out the name of the format, for example, Excel Workbook (.xlxs). 

Don’t use braces {} or brackets [] in place of parentheses.

Parenthetical text can be a word, a fragment, or multiple sentences. Your sentence should make sense when you read it out loud without the parenthetical text.

If a sentence ends with a parenthetical that’s only part of a larger sentence, the period or question mark goes outside the closing parenthesis (as we have done with this sentence). 

If the parenthetical itself is a whole sentence, the period or questions mark goes inside the parenthesis (why would you do it any other way?).

Use parentheses to offset acronyms and abbreviations and their spelled-out versions on first use.

  ✓  Company-specific/less common acronym: Application Lifecycle Management (ALM)

  ✓  Common acronym: HTML (Hypertext Markup Language)

Note that common acronyms such as HTML may be more familiar than their spelled-out versions and might not require the parenthetical. 

Think about your audience and if there's any doubt they may not understand a term, it's acceptable to put the spelled-out version in parentheses after the acronym on first use.

See also: Abbreviations and acronyms and check the A–Z word list or Merriam-Webster(opens external page).