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Punctuation Rules for Periods in Written English

Punctuation Rules for Periods in Written English

Punctuation marks are a convention of written language that ensure the clarity of writing. There are five punctuation rules for using periods as punctuation marks in written American English:

  • Sentence endings
  • Decimal points
  • Abbreviations
  • Computer files and Internet addresses
  • Play and poetry citations

The following sections explain and provide examples of the punctuation rules for periods in written English.

Ending Sentences

PeriodUse a period at the end of most declarative sentences. A declarative sentence is a sentence that makes a statement. For example:

  • The pumpkins molded in the garden.
  • A noun has traditionally been defined as a word that names person, place, or thing.
  • Cartoon Network is a popular television channel.
  • My son ate all the cookies.

Use a period at the end of an imperative sentence that does not convey strong emotion. An imperative sentence is a sentence that makes direct commands, expresses requests, and grants or denies permission. For example:

  • Please open the window.
  • Bring a dessert to dinner tomorrow night.
  • Wash your hands before eating.
  • Press the blue button to print.

In general, most sentences in written English end with a period.

Decimal Points

Use a period as a decimal point to mark the boundary between an integral from the fractional part of a number. An integral is the whole number on the left side of a decimal point. A fraction is the number on the right side of a decimal point. For example:

  • 1.5
  • 3.14
  • 13.375
  • 8,652.3

Use a period to separate dollars from cents when writing about money in American English. For example:

  • $1.99
  • $0.25
  • $175.03
  • $2,675.17


Use a period with most abbreviations including abbreviated titles, times, and words. For example:

  • Barty Crouch, Jr.
  • Ms. Johnson
  • 5:00 p.m.
  • c. (circa)
  • Apr. (April)

Do not use a period with state or most organizational abbreviations. For example:

  • LA (Louisiana)
  • OR (Oregon)
  • NASA (National Aeronautics and Space Administration)
  • NATO (North Atlantic Treaty Organization)

Note that some style guides require slightly different rules for the use of periods with abbreviations.

File Names and Internet Addresses

Use a period to separate the name of a computer file from the file extension. A file extension identifies the type of file that a computer file is. For example:

  • englishpunctuationrules.doc
  • puppy.jpg
  • bluewaterfall.css
  • fireflies.mp3

Use a period to separate elements of Internet addresses including URLs and email addresses. For example:


Drama and Poetry Citations

Use a period to separate book, line, act, and scene elements in drama and poetry citations. Place a period between the acts and scenes of a prose play and between the acts, scenes, and lines of a verse play. For example:

  • For Whom the Seagulls Fly 1.3 (act.scene)
  • Contradiction in a Square Degree 2.2 (act.scene)
  • Faust 1.1.125 (act.scene.line)
  • A Midsummer’s Night Dream 2.1.365 (act.scene.line)

Place a period between the books and lines of a poem. For example:

  • Iliad 2.45 (book.line)
  • The Faerie Queene 4.654 (book.line)
  • Song of Myself 1.34-38 (book.lines)
  • Aurora Leigh 9.12-35 (book.lines)

Punctuation is a convention of writing that help readers more clearly understand written language. Periods perform five basic functions in written American English: end sentences, as decimal points, abbreviations, electronic files and Internet addresses, and poem and drama citations.


Faigley, Lester. 2003. The Brief Penguin Handbook. New York: Pearson Longman.
Gibaldi, Joseph. 2003. MLA Handbook for Writers of Research Papers. 6th ed. New York: The Modern Language Association of America.

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