Using Periods (Full Stops)

How to Use Periods Correctly

A period, also known as a full stop, is an essential punctuation mark in written English. The main role of a period is to mark the end of a sentence. The proper use of periods helps improve the clarity and readability of writing. Here are four common uses for periods:

(1) Use a period at the end of a declarative sentence.

A declarative sentence is a sentence that makes statement. For example:
  • The sky is blue.
  • The cat sat on the mat.

(2) Use a period at the end of a mild imperative sentence.

An imperative sentence is a sentence that gives an order. (A mild order ends with a period. A forceful order ends with an exclamation mark.) For example:
  • Please keep off the grass.
  • Tidy your room before lunch.

(3) Use a period to end a contraction.

A contraction is a contracted version of a word. For example:
  • It is time for Mr. Jones and Capt. to leave.
  • Prof. Jackson is outside.

(4) Use periods in lowercase abbreviations.

For example:
  • The story is on every major news channel, e.g., CNN and BBC.
  • (Using periods in abbreviations is usually optional. You should follow your local convention and copy any official versions of abbreviations. The most common practice is to use periods for lowercase abbreviations but not uppercase ones.)

Table of Contents

  • Examples of Periods at the End of Declarative Sentences
  • Examples of Periods at the End of Imperative Sentences
  • Examples of Periods in Contractions and Abbreviations
  • Why Periods (Full Stops) Are Important
  • Test Time!
periods (full stop) usage
A period is also used at the end of a sentence fragment. A sentence fragment is a non-sentence that is punctuated like a sentence. For example:
  • The sky is a menacing blue. Almost a navy blue. It's quite unnerving.
  • (Sentence fragments are common in poetry, verse, and storytelling. They are considered non-standard, which is why this use of a period has not been listed in the "4 main uses" above.)
Read more about sentence fragments.

Examples of Periods at the End of Declarative Sentences

Periods are used at the end of declarative sentences. (A declarative sentence states a fact.)
  • Last words are for fools who haven't said enough. (Revolutionary socialist Karl Marx)
  • I never forget a face, but in your case I'll be glad to make an exception. (Comedian Groucho Marx)
  • I am a Marxist of the Groucho tendency. (Anon)
Read more about declarative sentences.

Examples of Periods at the End of Imperative Sentences

Periods (full stops) are used at the end of imperative sentences that are not forceful enough to warrant an an exclamation marks. (An imperative sentence gives a command.)
  • Write again soon.
  • Advise no one to go to war or marry. (Spanish proverb)
  • Avoid a cure that is worse than the disease. (Greek storyteller Aesop)
Read more about imperative sentences.

Examples of Periods in Contractions and Abbreviations

Here are some examples of periods in abbreviations:
  • a.m.
  • etc.
  • Prof. Munro
Read more about periods in abbreviations. Read more about the US and UK conventions for ending a contraction with a period (full stop).

Why Periods (Full Stops) Are Important

It's just a small dot, but the period can be a real grammar villain. Here are six good reasons to think more carefully about periods.

(Reason 1) Once you've written a sentence, put a period. Don't put a comma and write a new sentence.

The period is responsible for the most common mistake corrected by proof-readers: the run-on sentence. A run-on sentence is typically caused by writing a sentence, incorrectly putting a comma and then writing another sentence.

Your idea is likely to consist of several sentences. Once you've satisfied the criteria for a sentence, you must end it correctly (usually with a period, a question mark, or an exclamation mark) even if you haven't finished making your point. It's okay to make your point using several sentences. (Don't forget that a sentence is grammatically complete, expresses a complete thought and contains a subject and a verb (even if one is implied). In the examples below, all the commas should be periods.
  • Guard your cheese pies, they're Lee's favourite. wrong cross
  • This suspense is terrible, I hope it will last. wrong cross (Irish poet Oscar Wilde)
  • I have a wife and kids, eat them! wrong cross (Homer Simpson)
  • The answers to life's problems aren't at the bottom of a bottle, they're on TV. wrong cross (Homer Simpson)

(Reason 2) Use a period (not a question mark) to end a declarative sentence that contains an indirect question.

A declarative sentence can include an indirect question. Do not be tempted to use a question mark. Both of these examples should end with periods.
  • She asked if it were true? wrong cross
  • I wonder if a soldier ever does mend a bullet hole in his coat? wrong cross (Pioneering nurse Clara Barton)
Read more about indirect questions.

(Reason 3) Be confident with using and not using periods with contractions.

Writers are often unsure whether to write "Mr Smith" or "Mr. Smith," or "Dr Jones" or "Dr. Jones." If you're following US convention, there's nothing to think about. Always put a period (e.g., "Mr. Smith," "Dr. Jones," "Prof. Munro").

It's not so simple for Brits. If the last letter of the contraction (e.g., "Mr") is the same as the last letter of the whole word (e.g., "Mister"), don't use a period (full stop). If the last letters are different, use one.
  • Mister > Mr
  • (The last letters are the same.)
  • Professor > Prof.
  • (The last letters are different.)
  • Read paras 1-9 and para. 23.
  • (The last letters of paras and paragraphs are the same, but the last letters of para and paragraph aren't.)
If you're following the UK convention, no one is going to hang you for adopting the US method. Lots of Brits do. Nevertheless, it's comforting for Brits to know the UK rule. It removes the guesswork. It'll feel good to know you could justify the period in "Rev. Bloggs" or the absence of one in "Revd Bloggs." (In the UK, both "Rev." and "Revd" are acceptable contractions of "Reverend.")

(Reason 4) Don't use two periods at the end of a sentence.

If a sentence ends with an abbreviation that ends with a period, don't use a period to mark the end of the sentence. In other words, one period suffices.
  • I need milk, bread, cheese, etc.. wrong cross
  • She moved from I.T.V. after an irresistible offer from the B.B.C.. wrong cross
  • (Logically, these are correct, but they're too unwieldy.)
Question marks and exclamation marks are not affected.
  • Will the judge find in favour of the B.B.C.? correct tick
  • (This is correct, but it looks scruffy. To avoid this, use BBC instead of B.B.C.)
  • You were meant to be here at 4 o'clock a.m. not p.m.! correct tick
The only exception to doubling up end marks with periods is the ellipsis (...).
  • Standing tall and with the Lord's Prayer mumbling across our lips, we entered the chamber...."
  • (This ends in four dots: three for the ellipsis and one to end the sentence.)

(Reason 5) Write uppercase abbreviations without periods and lowercase ones with.

Often, you have a choice whether to write your abbreviation with periods. In other words, you can write C.N.N. or CNN, or e.g. or eg. Whatever format you choose, be consistent.

By far the most common format is to write uppercase abbreviations without periods (for example, CNN, LRS) and to write lowercase abbreviations with periods (for example, a.m., e.g.).

There's just one rule that might trump your striving for consistency with formatting abbreviations. If the abbreviation is a company name, copy the format the company uses. (Hey, don't expend too many calories worrying about this point. Very few companies use periods in their names. I mean almost none.)

(Reason 6) Be careful not to omit the last period of an abbreviation.

If you choose to use periods in your abbreviations, don't forget to give the last letter a period too.
  • "M.O.T While You Wait" wrong cross
  • (This is a common sign in the UK.)
  • "M.O.T. While You Wait" correct tick

Key Points

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This page was written by Craig Shrives.