In the 3rd century BC, Aristophanes of Byzantium invented a

(for, and, nor, but, or, yet, so)

To separate the names of smaller geographic units from the names of larger units

Example:Denver, Colorado, is called the Mile High City.

To separate dates, if the order is month-day-year. If in the middle of a sentence, use another comma after the year

Example: Kennedy was assassinated November 22, 1963, in Dallas, Texas.

To separate names used in direct address or other isolates such as yes, no, and thank you from the rest of the sentence

Examples:

Tell me, Eddie, for whom did you vote?

No, I am not voting for myself.

To separate tag questions from the rest of the sentence

Example:

You did that on purpose, didn't you?

To separate words and abbreviations that introduce an example or an illustration, including namely, that is, to wit, i.e., e.g., for example, and for instance

Example:

Many of my friends, for example, Bruce, Donald, and Harry, like to play pinochle.

independent clauses

To separate direct quotations from the phrase identifying the speaker

When there are no dependent clauses in the same sentence an independent clause, the independent clause is a simple sentence. For example:

I like coconut macaroons.

(This is an independent clause and simple sentence.)

I like coconut macaroons even though I dislike coconut.

(This is an independent clause and a dependent clause. This is a complex sentence.)

Read more at http://www.grammar-monster.com/glossary/independent_clause.htm#gOGzqxsjrs6W2fmf

Example: John said, "The fishing is great in Blackwater River."

To separate two independent clauses joined by a coordinating conjunction

Examples:

I have known the applicant for ten years, and I am pleased to recommend her for this job.

Either I will meet you in the lobby, or I will come to your office.

John said that "the fishing is great."

Dates

Quotations

Tag Questions

In the 3rd century BC, Aristophanes of Byzantium invented a system of single dots (distinctiones) that separated verses (colometry), and indicated the amount of breath needed to complete each fragment of text, when reading aloud.[1] The different lengths were signified by a dot at the bottom, middle, or top of the line. For a short passage (a komma), a media distinctio dot was placed mid-level ( · ). This is the origin of the concept of a comma, although the name came to be used for the mark itself instead of the clause it separated.

. The virgule, originally placed high, sank to the baseline and developed a curve—it turned, in fact, into a modern comma. The Venetian editor and printer Aldus Manutius (Aldo Manuzio; died 1515) made improvements in the humanistic system, and in 1566 his grandson of the same name expounded a similar system in his Orthographiae ratio (“System of Orthography”); it included, under different names, the modern comma, semicolon, colon, and full point, or period. Most importantly, the younger Aldo stated plainly for the first time the view that clarification of syntax is the main object of punctuation.