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Commas and Numbers
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American and British Differences
 

A comma ( , ) is a punctuation mark.

It has the same shape as an apostrophe or single closing quotation mark in many typefaces, but it differs from them in being placed on the baseline of the text.

Some typefaces render it as a small line, slightly curved or straight, some like a small filled-in number 9. It is used in many contexts, principally for separating things. According to the Oxford English Dictionary , the word “comma” comes directly from the Greek komma, which means “something cut off” or “a short clause”.

  1. It is used to mark off separate elements in a sentence:
    • introductory words and phrases
      • “Once upon a time, I didn't know how to use commas.”
    • parenthetical phrases
      • The parenthetical phrase is an important, often misunderstood use. It is often used for thought interruptions. Information that is unnecessary to the meaning of the sentence must be set off and enclosed by commas. If the information is necessary, no commas should be used.
        • For example, the sentences “I cut down all the trees, which were over six feet tall” and “I cut down all the trees that were over six feet tall” look similar but in fact have very different meanings. In the first sentence, all the trees were cut down, and a detail (that they were over six feet tall) is added. In the second, only some trees were cut down - those over six feet tall; there may have been shorter trees, too, which were not cut down. In the first case, “which were over six feet tall” is set off by a comma because it is a non-restrictive clause (i.e., its removal doesn't alter the meaning of the sentence). In the second, “that were over six feet tall” is a restrictive clause and takes no comma (because if you left it out, the sentence would then say that all the trees were cut down, not just the ones over six feet).
      • Parenthetical phrases in sentences include explanations of noun subjects (“The film's star, Tom Cruise, said...”), years following dates (in American use) and months (“On December 7, 1941, Pearl Harbor...” [see #7 below]), and states following cities (“John F. Kennedy was shot in Dallas, Texas, in 1963”). In each case, the parenthetical (as if in parentheses) is both preceded and followed by a comma.
  2. The comma is also used to separate two independent clauses (a group of words that can function as a sentence) that are joined by a coordinating conjunction (“for”, “and”, “nor”, “but”, “or”, “yet”, and “so“ when they are used to connect; the acronym “fanboys” can be used as a memory aid).
    • “I passed the test, but he failed.” - “I passed the test” and “He failed” can function as separate sentences
    • “I walked home and left shortly after.” - Although “I walked home” is independent, “left shortly after” is dependent on the first part of the sentence
  3. Commas are used to separate items in lists. However, if the individual items in the list also contain commas, the list should be separated by a semicolon(“;”)
    • A comma before the word “and” or “or” in a list of more than two things is called a serial comma, or an Oxford comma:
      • “We had tea, biscuits, and cake.”
      • It is so named because its usage is recommended in the style guide of the Oxford University Press.
      • Although the Oxford comma is not always used, it is essential in certain sentences to avoid ambiguity.
        • “I spoke to the boys, Sam and Tom.” - “The boys” refers to Sam and Tom.
        • “I spoke to the boys, Sam, and Tom.” - “The boys”, Sam, and Tom are separate units; thus four or more people were spoken to in all.
  4. A comma may be used in a sentence before a quotation.
    • Mr. Kershner says, “You should know how to use a comma.”
      • An exception can be made if the sentence is structured as a single, fluid event: Mr. Kershner told me that I “should know how to use a comma.” Also, some style guides prefer the use of a colon.
  5. Commas are used in some (mostly English-speaking) countries to present large numbers in a more readable form.
    • “1,000”
    • “1,000,000”
  6. Commas are used when writing names that are presented last name first.
    • Bond, James
  7. Two commas used when writing the date in the following forms:
    • American English: September 11, 2001, was a momentous day.
    • British English: Tuesday, 11 September 2001, was a momentous day.
  8. A comma is written in an address between the city and the state:
    • My dog's masseuse lives in New York, NY.
  9. Fowler's Modern English Usage demonstrates an optional use of commas with two sentences, differing only by a comma:
    • “The teacher beat the scholar with a whip”. A simple description.
    • “The teacher beat the scholar, with a whip”. Expression of outrage.
      • This second example does not specifically require a comma, and is considered nonstandard usage.

The comma is easy to misuse in multiple ways; see comma splice.

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