What is a period?
The period (also known as a full stop, especially in British English) is a punctuation mark ( . ) primarily used to indicate the end of a sentence. It appears as a single dot on the bottom line of the text, and it comes immediately after the last word of the sentence without a space.
There are also several minor uses for the period, which we’ll look at later in this section.
The most common use of periods is to punctuate the end of sentences. This occurs with almost all the types of sentences we use. For example:
Note that if we wanted to indicate an especially strong emotion in any of the above sentences, or else indicate that the speaker is shouting, we would typically use an exclamation point instead. For instance:
- “I run three miles every morning!”
- “I could study better if you turned down the music!”
- “Please start the car!”
However, this is based solely on the intention of the writer; the form of the sentence itself doesn’t require that an exclamation point be used instead of a period.
Interrogative sentences (Questions)
The only type of sentence in which a period is not used is the interrogative sentence (a sentence that asks a question). These sentences always use question marks rather than periods, as in:
- “Would you like a bite of my sandwich?”
- “Where is the closest supermarket?”
- “Did you sleep well?”
Using periods with quotation marks
If a sentence uses quotation marks to indicate something that another person has said, and the quotation marks appear at the end of the sentence, the period can appear in one of two places.
In American English, the punctuation used at the end of direct speech always appears within the quotation marks. For example:
- The CEO said, “This is a great day for the company.”
- Our five-year-old said that when she grows up she wants to be a “doctor and a pop star and a ballerina.”
However, in British English (which also uses different rules for quotation marks), periods that end quotations are usually placed outside the final quotation mark, as in:
- The CEO said, ‘This is a great day for the company’.
- Our five-year-old said that when she grows up she wants to be a ‘doctor and a pop star and a ballerina’.
One vs. two spaces after a period
When we begin a new sentence after another one has ended, we generally use a single space between the period of the first sentence and the first word of the second sentence.
However, at one point, it was standard practice to add two spaces after the end of a sentence. This trend arose out of the use of manual typewriters (in which the monotype font created a lack of space between periods and the first letter of the next sentence). With the advent of word processor applications for computers, this problem was eliminated, but some writers today persist in adding two spaces after a period.
While this continues to be a point of contention for some, the definitive answer is that it is not only preferable but correct to use only one space after the period of a sentence. This is a point on which all modern grammar and style guides (including this one) agree: When beginning a new sentence after a period, only use a single space.
Periods with abbreviations
While the period is most commonly used to mark the end of a sentence, it is also used to mark abbreviations. In addition to standard abbreviations (words that are shortened by omitting one or more letters), there are also three sub-categories that can use periods: initials, acronyms, and initialisms.
However, unlike its use in punctuating the end of a sentence, the use of a period with abbreviations is not as strict or consistent, often depending on the location (i.e., American English vs. British English) and preference of the writer, publisher, or style guide.
We often use periods to indicate when a word has been abbreviated (that is, has had letters omitted from it) in a sentence. This period always appears at the very end of the abbreviated word. For example:
- “Please cont. to page 41 for further instructions.” (abbreviation of continue)
- “Dr. Davis has been of immense service to the hospital.” (abbreviation of Doctor)
Note that when an abbreviation ends a sentence, the period that shortens the word also marks the end of the sentence—that is, we do not use a second period. If another punctuation mark (such as a comma or question mark) is used after an abbreviated word, it comes directly after the period. For example:
- “Our offices are open each week Mon.–Fri.” (abbreviations of Monday and Friday)
- “My new house is on Lilac Ave., just across from the old courthouse.” (abbreviation of Avenue)
- “Are you returning to work in Feb.?” (abbreviation of February)
Titles in British English
Formal titles, such as Mr., Mrs., or Dr., are actually abbreviations, but we usually only use them in their contracted forms in writing. Because they are so common, the period is usually left out in British English, as the abbreviation does not need to be indicated by punctuation to be understood. It’s therefore common to read “Mr Jones,” “Mrs Smith,” or “Dr Casey” (for example) in writing that uses British English styles.
In American English, however, these abbreviated titles are still punctuated with periods.
Initials of names
Initials are a kind of abbreviation of people’s names formed from the first letter of each part of the name (first, middle, and/or last name). We usually mark each initial with a period. For example:
- “Martin S. Smith, the renowned physicist, will be speaking here next month.”
If sequential parts of someone’s name are made into initials, then a space will usually come after the period of the initial. For example:
- “I love the writing of J. R. R. Tolkien.”
However, while some style guides recommend this spacing between letters, other style guides recommend not including spaces, as in:
- “I love the writing of J.R.R. Tolkien.”
Whether to use spaces after the periods of initials is largely a matter of personal preference, but be sure to check the recommendations of your school’s or employer’s style guide.
(Note: It is also not uncommon to see multiple initials used without either periods or spaces, as in “JRR Tolkien,” but this usually occurs in less formal writing.)
Acronyms and Initialisms
Similar to initials, acronyms and initialisms are abbreviations of multiple words using just their initial letters.
Acronyms are distinguished by the fact that they are read aloud as a single word. Because of this, they are usually (but not always) written without periods. In some cases, the acronym has become so common that the letters aren’t even capitalized anymore.
- “Scientists from NASA have confirmed the spacecraft’s location on Mars.” (acronym of “National Aeronautics and Space Administration”)
- “The officer went AWOL following the attack.” (acronym of “Absent Without Leave”)
- “I need those documents finished A.S.A.P.” (acronym of “As Soon As Possible”; also often written as ASAP, asap, and a.s.a.p.)
- “His scuba equipment turned out to be faulty.” (Scuba is actually an acronym of “self-contained underwater breathing apparatus,” but it is now written as a regular word.)
Initialisms are formed in the same way as acronyms, but they are spoken aloud as individual letters rather than a single word. (However, because they are so similar in appearance to acronyms, initialisms are very often simply referred to as acronyms.)
Like acronyms, it is most common to write initialisms without periods. However, in American English, it is also common to include periods between the letters of some initialisms. This varies between style guides, and it is generally a matter of personal preference; whether you use periods in initialisms or not, be sure to be consistent.
Here are some examples of common initialisms (some with periods, some without):
- “I grew up in the US, but I’ve lived in London since my early 20s.” (initialism of “United States”)
- “It took a long time, but I’ve finally earned my Ph.D.” (initialism of “Philosophiae Doctor,” Latin for “Doctor of Philosophy”)
- “I need to go to an ATM to get some cash.” (initialism of “Automated Teller Machine”)
- “The witness claimed to have seen a U.F.O. fly over the field last night.” (initialism of “Unidentified Flying Object”)
The decimal point
One final common use of the period is as a decimal point—the dot that separates a whole number from its decimals. For example:
- “I can’t believe he sold you that car for $450.50!”
- “Today’s Special: Two-course meal and a bottle of wine for two, only $45.95.”
- “We found that X equals 43,456.21.”