Use a period to close sentences that make statements or give mild commands.

All books are either dreams or swords.

—Amy Lowell

Don’t use a fancy word if a simpler word will do.

—George Orwell, “Politics and the English Language”

A period also closes indirect questions, which report rather than ask questions.

I asked how old the child was.

In American English, periods are used with most abbreviations. However, more and more abbreviations are appearing without periods.

Some abbreviations rarely if ever appear with periods. These include the postal abbreviations of state names, such as FL and TN, and most groups of initials (GE, CIA, AIDS, YMCA, UNICEF). If you are not sure whether a particular abbreviation should include periods, check a dictionary or follow the style guidelines you are using for a research paper. (For more about abbreviations, see Chapter 42.)

Do not use an additional period when a sentence ends with an abbreviation that has its own period.