There are multiple uses for commas, such as: separation of clauses, parenthetical phrases, between adjectives, before quotes, in quotes, in dates, in geography, in numbers, and in names, etc.
Separation of Clauses
Commas are often used to separate clauses. In English, a comma is generally used to separate a dependent clause from the independent clause if the dependent clause comes first: After I brushed my teeth, I used mouthwash. (Compare: I used mouthwash after I brushed my teeth.) A relative clause takes commas if it is non-restrictive, as in Pamela often wears high-heeled shoes, which have three-inch heels. (Without the comma, this would imply that only some of the heels – those over three inches tall – were worn. Some consider the word "that" to be preferable when such a meaning is desired: "Pamela often wears high-heeled shoes that have three-inch heels.") Two independent clauses joined by a coordinating conjunction (for, and, nor, but, or, yet, so) or having a complementary relationship may or may not be separated by commas, depending on preferred style, or sometimes a desire to overcome ambiguity. While many style guides call for commas, many authors omit them, particularly with short sentences.
Parenthetical Phrases
Commas are often used to enclose parenthetical words and phrases within a sentence (i.e. information which is not essential to the meaning of the sentence). Such phrases are both preceded and followed by a comma, unless that would result in a doubling of punctuation marks, or the parenthetical is at the start or end of the sentence. The following are examples of types of parenthetical phrases:
Introductory Phrase: Once upon a time, Rapunzel let down her hair.
Address: Rapunzel let down her hair, Your Honor.
Interjection: Rapunzel let down her hair, I swear!
Aside: Rapunzel, if you don’t mind my telling you this, let down her hair.
Appositive: Rapunzel, a jaded and bitter woman, let down her hair.
Absolute Phrase: Rapunzel, her lips emitting a sigh, let down her hair.
Free Modifier: Rapunzel, leaning out the window, let down her hair.
Resumptive Modifier: Rapunzel let down her hair, hair of gold.
Summative Modifier: Rapunzel let down her hair, as no one had before.
Between Adjectives
A comma is used to separate coordinate adjectives; that is, adjectives that directly and equally modify the following noun. Adjectives are considered coordinate if the meaning would be the same if their order were reversed or if and were placed between them. For example:
The dull, incessant droning but the cute little cottage. The sweet furry dog suggests there are furry dogs (one of which is sweet), while the sweet, furry dog does not carry this connotation.
Before Quotes
A comma is used to set off quoted material that is the grammatical object of an active verb of speaking or writing, as in Mr. Malburg says, "You should know how to use a comma." Quotations that follow and support an assertion should be set off by a colon rather than a comma.
In Dates
When a date is written as a month followed by a day followed by a year, a comma separates the day from the year: December 7, 1941. This style is common in American English. Additionally, most style manuals, including The Chicago Manual of Style and the AP Stylebook, recommend that the year be treated as a parenthetical, requiring a second comma after it: "Feb. 14, 1987, was the target date."
In Geographical Names
Commas are used to separate parts of geographical references, such as city and state (Dallas, Texas) or city and country (Kampala, Uganda). Additionally, most style manuals, including The Chicago Manual of Style and the AP Stylebook, recommend that the second element be treated as a parenthetical, requiring a second comma after: "The plane landed in Kampala, Uganda, that evening." The United States Postal Service encourages the writing of address labels without any punctuation (and all in capital letters).
In Numbers
In representing large numbers, English texts use commas (or spaces) to separate each group of three digits. This is almost always done for numbers of six or more digits, and optionally for five (or even four) digits. However, in many other languages (and in South Africa) the comma is used as a decimal separator, equivalent to the decimal point. In addition, the comma may not be used for this purpose at all in some number systems, e.g. the SI writing style, and a space may be used to separate every three digits instead.
In Names
Commas are used when writing names that are presented surname first: DelVillano, Ron. They are also used before many titles that follow a name: Ron DelVillano, Ph.D. Commas, however, would be used incorrectly if put after a clause, then followed by a name.
Coordinating Conjunctions
A coordinating conjunction can join two main clauses that a writer wants to emphasize equally. The pattern for coordination looks like this: main clause +, + coordinating conjunction + main clause. (cc) "It's tough to stay married. My wife kisses the dog on the lips, yet she won't drink from my glass." (Rodney Dangerfield) Coordinating Conjunctions include: For, And, Nor, But, Or, Yet, & So. “THINK FANBOYS!”
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