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
Use 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
Abbreviations
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:
- https://www.facebook.com/ParentingPatch/
- http://www.google.com/
- help@suite101.com
- email@email.com
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.
References
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.
