Most Common Rules of Usage Regarding the Comma
- To Join Independent Clauses Into Compound Sentences
- To Join Introductory Clauses, Phrases, or Subordinate Clauses That Come Before the Main Sentence
- In Serial Lists Containing Three or More Subjects, Phrases, or Clauses
- In Places, Titles, and Dates
- In Parenthetical or Non-Essential Elements of a Sentence
- To Coordinate Adjectives
- Before Dialogue Introduced by Quotations
A. To Join Independent Clauses Into Compound Sentences
I. An independent clause is a sentence that acts as a single, complete thought.
- Example: “The ancient Mesopotamians developed a system of canals.”
- Example: “Mesopotamian canals created severely salinized soil.”
Both of these sentences, while related in theme, function as complete, independent thoughts. They are independent clauses.
II. A dependent clause is a sentence that relies in some meaningful way on another clause for clarity.
- Example: “To irrigate their fields and crops.”
- Example: “Deposited extra sediment that was not properly drained.”
These clauses depend on another statement for clarity and meaning. They are dependent clauses.
III. A compound sentence consists of two independent clauses joined by a comma and linked by a conjunction.
These parts of the sentence can be seen as separate but still function as one complete thought. If divided, they risk becoming two choppy sentences, so the comma and conjunction are vital in avoiding that problem.
- Example: “The ancient Mesopotamians developed a system of canals, but these canals created severely salinized soil.”
- Example: “The ancient Mesopotamians developed a system of canals, which led to severely salinized soil.”
In the second example, the subject is removed for simplicity because it has now been compounded with that of the first clause. Either example is correct.
IV. To test if a sentence has two independent clauses that should be joined by a comma and a conjunction, identify the main thought in each part of the sentence and determine if it can stand alone or if it needs the information in the adjacent clause for clarity and meaning. Generally, you will end up with two clauses, or parts, of the sentence. If the second clause depends in some way on the first clause (usually the subject or the verb), it is not a complete (and therefore independent) thought. In this case, there is no need for a comma before the conjunction.
- Example: “The ancient Mesopotamians developed a system of canals in order to irrigate their fields and crops.”
- Example: “Mesopotamian canals created severely salinized soil because they brought in extra sediment that was not properly drained.”
If the first clause depends in some way on the second clause, it acts as an introductory clause and should be separated from the main sentence by a comma (see next section).
- Example: “In order to irrigate their fields and crops, the ancient Mesopotamians developed a system of canals.”
B. To Join Introductory Clauses, Phrases, or Subordinate Clauses That Come Before the Main Sentence
An introductory clause or phrase acts as a qualifying or clarifying statement about the main sentence. Think of it as an addition to the main thought of a sentence. The introductory language is usually a dependent statement (see section A). In order to indicate to the reader where the main thought begins and ends, and thus clarify the writer's point, a comma should be used after the introductory language. Again, these words are often contained in a dependent phrase anyway.
- Example: “By 1700 BC, wheat could not be grown in Mesopotamia due to salinization of the soil."
This sentence has both an introductory clause, “By 1700 BC,” and a dependent clause, “due to salinization of the soil.” The main sentence, “wheat could not be grown in Mesopotamia,” is being amended or clarified by these clauses.
II. Phrases are commonly used as introductions to a sentence.
Words like 'however,' 'indeed,' 'therefore;' phrases like 'on the one hand,' 'in particular,' 'for example,' and 'in the meantime' are examples of introductory language that ought to be concluded with a comma when they begin a sentence.
- Example: "However, the Fertile Crescent may not actually have been as fertile as the name suggests."
III. A clause can be “tested” for its function by moving the language in question to the end of the main clause and asking, "does it still work in the sentence?"
- Example: “The Fertile Crescent may not have actually been as fertile as the name suggests, however.”
Notice here that the introductory word was moved to the end of main sentence in order to test its function. Because it still makes the point and clarifies the main sentence in this new position, we can determine that it is an introductory phrase and thus that it ought to be concluded with a comma if it is placed at the beginning of the main sentence, or preceded by a comma when it is placed at the end.
- Example: “Wheat could not be grown in Mesopotamia by 1700 BC due to salinization of the soil.”
Notice that this looks like a compound sentence with a dependent clause at the end—like the example from Section A .IV (it actually has two dependent clauses tacked on—“by 1700 BC” and “due to salinization of the soil”). The introductory phrase needs a comma if it will be placed at the beginning of the main thought, but may not if it is placed at the end. This is because it can function as either introductory language or an essential statement (see below). If it comes later in the sentence, the conjunction will suggest whether to use a comma or not.
If the introductory sentence, when placed at the end of the main clause, no longer properly introduces, clarifies, or amends the main clause, you may have two independent clauses. Follow the rules from section A for dealing with independent clauses.
- Example: “The ancient Mesopotamians developed a canal system, and the soil in the area became severely salinized.”
- Example: “Mesopotamian soil became severely salinized, and the ancient Mesopotamians developed a canal system.”
In the second example, the two independent clauses indicate cause and effect and so cannot be reversed without destroying the meaning of the sentence. Hence neither functions as a dependent or introductory clause, and the comma usage must follow the rules governing compound sentences.
IV. Watch out for sentences that start with introductory statements, but then introduce a main clause that looks like a serial list.
This is precisely the type of error the Oxford comma is designed to fix, but it means that special attention must be paid to places where the Oxford looks like it should be used but isn't.
- Example: “Because of its unique geography, boundaries and inhabitants tended to be transient in southern Mesopotamia.”
Directly following the comma behind "geography" are two more objects--boundaries and inhabitants--which leads the reader to think a serial list is coming. Thus I might have to stop reading at "tended" because I was expecting a verb that relates to the entire series in the list I thought I just read (something that speaks to all of "geograpy, boundaries, and inhabitants"). Because it doesn't, I have to back up, reread the sentence, and understand it in a different way--the way you actually meant it. But this process confuses the point and puts your style in front of your argument. However, the only way to fix this issue is to change the sentence--changing or removing a comma will only turn the sentence into something else and cause additional problems.
- Example: “The boundaries and inhabitants of southern Mesopotamia tended to be transient because of the area's unique geography.”
By seperating the subject (boundaries and inhabitants) from the object (geography), you now don't have the issue with list confusion. Since commas are the problem in this type of sentence structure, adding or removing them won't help. But fixing the structure does, and it's an easy fix to apply.
C. In Serial Lists (Containing Three or More Subjects, Phrases, or Clauses)
I. Commas clarify items in a list that are connected by the concept of the thought, but otherwise disconnected from each other.
In this case, always separate these items by commas—up to and including the last item. This is known as the Serial or Oxford comma.
- Example: “Mesopotamia is better understood not only as a place but as a series of cultures that exhibited common beliefs, patterns of development, conquest and defeat, and who lived and died in an area roughly centered in the land between the rivers.”
- Example: “Caesar was bold, brave, and successful.”
II. Some departments and publishers will specifically request the omission of the Oxford comma.
Always follow the guidelines of the party for whom you are writing. When it is not explicitly stated, however, employ the Oxford (see section 33 for rules on consistency).
D. In Places, Titles, and Dates
Commas are used to separate clarifying information in the proper names of geographic places, proper titles, and certain forms of date. The rules governing this are similar to those for non-essential and parenthetical comma use—the comma is used to separate part of the information that clarifies the main point, but is not necessarily important in its own right.
- Example: “Dr. Mark Damen, Ph.D., teaches Ancient Near Eastern history at USU.”
The “Ph.D.” is not necessary in this statement, but it is part of the subject's official title and so is included and separated by commas.
- Example: “We took a vacation to Colorado Springs, Colorado, earlier this year.”
The state, “Colorado,” is not required in this sentence, but it clarifies the subject (Colorado Springs) and so is included for that purpose. Because it is parenthetical to the subject, however, it requires a preceding and concluding comma.
- Example: “Caesar was betrayed and murdered on March 15, 44 BC, in the theater of Pompey.”
“44 BC” is the clarifying part of the date, and so requires enclosure in commas.
E. In Parenthetical or Non-Essential Elements of a Sentence
I. Use commas to identify non-essential components.
Non-essential components of a sentence are pieces of information that add clarifying or qualifying information to the main subject of the sentence, but that do not change the basic meaning of the sentence or thought. These are statements that if removed entirely would not adversely affect the readability or meaning of the sentence as a whole.
Example: “The ancient shaduf, a tool designed to raise water between sources of differing elevations, allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”
The sentence would remain the same without the non-essential phrase between commas: “The ancient shaduf allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”
II. Identify between essential clauses and non-essential clauses.
Be careful about identifying between essential clauses (like those beginning with ‘that') and non-essential clauses (typically those beginning with ‘which'). The author must choose whether information is essential to the sentence or not, and then so indicate to the reader by employing the proper conjunction and punctuation. When in doubt, purposely use ‘that' for essential information and ‘which' for non-essential information.
- Example: “The ancient shaduf that raised water between sources of differing elevations allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.”
- Example: “The ancient shaduf , which raised water between sources of differing elevations, allowed the Mesopotamians to irrigate crops further away from the Tigris and Euphrates Rivers.
Notice that 'which' introduces a non-essential statement, while 'that' introduces an essential one. Since the statements are the same in these examples, the conjunction you choose and the comma placement tells the reader whether or not the statement you are making is essential to the overall thought.
“That” and “which” are also employed, along with “who,” in relative clauses. These can be treated the same as essential and non-essential (sometimes called restrictive and non-restrictive relative clauses), but these will be dealt with at greater length in section 34.
III. Enclose important yet inconsequential statements with commas.
Similarly, statements that serve an important purpose but do not necessarily modify or clarify the subject at hand ought to be enclosed in commas.
These statements are called parenthetical thoughts and are to be treated the same as non-essential clauses—since both components of a sentence can be removed without ruining the meaning of the main sentence, they need commas to clarify their comparative irrelevance to the main thought.
- Example: “His only thought, if he had one at all, was for his own well-being.”
- Example: “The USU Aggies, like most all fans of collegiate sports, are voraciously loyal to their team.”
F. To Coordinate Adjectives
Coordinate adjectives are words that apply to and describe some attribute of the same noun or subject. To do this effectively, however, the author must separate them by a comma—otherwise the reader may confuse the adjectives as a singular modifier and not a coordinate pair.
- Example: “The Tigris River ends near the Persian Gulf in a wide, swampy delta.”
This usage can be tested in two ways:
- Reverse the order of the adjectives. Is the meaning of the sentence still clear?
- Example: “The Tigris River ends near the Persian Gulf in a swampy, wide delta.”
- Place the conjunction 'and' between the adjectives. Is the meaning of the sentence still clear?
- Example: “The Tigris River ends near the Persian Gulf in a wide and swampy delta.”
If the sentence passes both tests, it uses coordinating adjectives. If not, it uses a singular adjective employing more than one word. These cannot be separated by commas or conjunctions without destroying their meaning.
- Example: “The Roman emperor Flavius was named for his ruby red hair.”
- Example: “The Roman emperor Flavius was named for red ruby hair.” (wrong)
- Example: “The Roman emperor Flavius was named for his red and ruby hair.” (wrong)
- Example: “The Assyrians employed fierce, brutal tactics against their opponents.”
- Example: “The Assyrians employed fierce, brutal, tactics against their opponents.” (wrong)
G. Before Dialogue Introduced by Quotations
I. Commas indicate an introduction to a phrase that it is active dialogue by a speaker.
This is different from a cited quotation (despite the use of quotation marks that are employed in both instances).
- Example: "Caesar, realizing too late who had betrayed him to the death, said to his old colleague, "Even you, Brutus?""
Notice that the comma here precedes the start of the dialogue.
II. Commas are not used when a quoted citation is employed within a sentence.
Note that quotations are not the same as dialogue. This is also true of translated text, which is indicated by quotations but not preceded by a comma.
- Example: “According to Damen, Mesopotamia is a cultural ideal that is "bounded by neither place nor time.""
Here, there is no comma before the quoted section because it operates within the sentence as a contiguous thought. The quotations here only indicate which parts of the language originated with someone else.
- Example: "The word Mesopotamia originated from the Greek and means 'land between the rivers.""
III. Pay attention to the conclusion of quotations or sections of dialogue.
When concluding either a quotation or a section of dialogue that is parenthetical, introductory, part of a main clause, or otherwise ought end in a comma, make sure to place the comma inside the quotation mark.
This goes for all other puncuation--periods, question marks, and the like--that end the quoted passage. The mistake is generally observed with commas more than these other marks, however.
- Example: "Caesar, who famously said, "let the die be cast," was killed in March of 44 BC."