Understand conventions for using commas and appreciate the likely effects of particular sentence lengths and patterns on reading comprehension.
Commas are like pawns in chess: They seem relatively insignificant and unobtrusive, yet they are actually very important. If properly placed, the lowly pawn can checkmate the king or, once it has reached the end of the board, become a more powerful piece. Commas play an extremely important role in ensuring that your documents are understandable. In fact, failing to insert a comma in the correct spot can cause considerable misreading (and subsequent embarrassment). Beyond a few special circumstances, there are six basic ways to use commas correctly.
Use Commas to Separate Adjacent Parallel Elements
As demonstrated by the following examples, a series is composed of three or more parallel elements, and the series can appear in the beginning, middle, or end of a sentence:
Stretching, warming up, and cooling down are important to a good exercise program.
All of the necessary qualities of a good assistant -- typing, shorthand, and patience -- she had in abundance.
The three qualities of a good introduction are context, purpose, and organization.
Editors and grammarians are in sharp disagreement about whether a comma should be placed before the last element in a series. The trend in the popular press is not to include the comma if the elements in the series are brief. However, many well-known stylists have persuasively argued that conjunctions connect and commas separate, so it is incorrect in their opinion to judge the comma as redundant punctuation before a conjunction such as "and." In addition, uninformed readers may perceive the last two elements in the series to be a compound if the comma is omitted. For example, placement of the comma before the word "and" in the following example makes it clear that flowering plants are not the same as ornamental bushes:
The landscaping contract includes several exotic plants, ornamental bushes, and flowering plants.
Occasionally, as dictated by your ear and the rhythm you hope to establish, you may want to insert a comma and forgo the and, as in this example:
We have a government of the people, by the people, for the people.
When you want to slow down the rhythm of your sentence and emphasize a point, you can replace all the serial commas with and or or:
He does not like shrimp or crayfish or lobster or anything that turns red when cooked.
If the abuse of the wetlands continues, we will be without waterfowl and fish and wildlife.
When you must present a long array of parallel elements in your documents, you can avoid listing them by grouping them into logical parts and punctuating accordingly, as demonstrated by the following examples:
Writing is painful and exhilarating, tedious and inspiring, chaotic and planned.
Human activities such as coal and oil burning, population growth and increased food demands, clearing and burning forests have caused increases in the release of carbon dioxide and methane.
Finally, note that coequal, consecutive coordinate adjectives that modify the same noun should generally be separated with commas:
Although he appears to have your best interests in mind, he truly is a competitive, combative, cantankerous boss.
However, you should not separate two consecutive adjectives with commas if the first adjective is modifying both the following adjective and noun as a unit, as illustrated below:
The competitive track star runs forty miles a week.
Use Commas to Join Two or More Independent Clauses
In most instances, place a comma between two sentences that are joined with a coordinating conjunction--and, but, or, for, nor, so, yet:
She was not sure if she had the necessary mathematical abilities to be an engineer, so she pursued a graduate degree in history.
He was surrounded by fifty people, yet he felt all alone.
You do not need to place a comma between two independent clauses if they are short and similar in meaning, provided that no misunderstanding will take place, as illustrated in the following example:
Some doctors advertise their services but many doctors find this reprehensible.
The absence of the comma in this sentence is acceptable; it is not necessary to prevent misreading.
Use Commas after Introductory Subordinate Clauses
To avoid confusion, use a comma after an introductory subordinate clause or phrase:
Because the costs of conducting research continue to increase, we need to raise our rates.
As the shrimp boats trawl, sea grass can collect on the trap door, allowing shrimp to escape.
According to the professor, rich women are more likely to have caesarean sections than poor women.
In keeping with the modern trend toward using as little punctuation as possible, some stylists believe that it is not necessary to place a comma after short introductory words (now, thus, hence) and phrases (In 1982 he committed the same crime). However, conservative style manuals still call for the comma, so you are better off playing it safe and placing a comma after introductory words and clauses.
Use a Comma After Conjunctive Adverbs and Transitional Phrases at the Beginnings of Sentences
Although our modern style calls for using as few commas as possible, you should generally place a comma after conjunctive adverbs and transitional words because they modify the entire sentence:
Nevertheless, we must push forward with our plans.
In other words, you're fired. Hey, I'm just kidding.
Because commas cause readers to pause in their reading, you want to use them sparingly. Although logic would suggest that it makes sense to follow coordinating conjunctions with commas, convention does not call for this usage unless the conjunction is followed by an introductory phrase. Thus, it would he inappropriate to write:
Yet, I think we should go ahead as planned.
When a short phrase follows the conjunction at the beginning of the sentence, however, it is appropriate--although not absolutely necessary--to place a comma after the conjunction:
Yet, as I mentioned yesterday, I think we should go ahead as planned.
Use Commas Around Nonrestrictive Parenthetical Elements
You should limit the number of times that you interrupt the flow of a sentence by placing modifying words between the subject and its verb. When you do introduce such appositives, participial phrases, or adjective phrases or clauses, you must determine whether the modifiers are restrictive or nonrestrictive. Essentially, restrictive modifiers add information that is essential to the meaning of the sentence, whereas nonrestrictive modifiers add information that is not essential. The best way to determine whether a modifier is restrictive or nonrestrictive is to see if taking it out changes the meaning of the sentence.
Restrictive: Lawyers who work for McGullity, Anderson, and Swenson need to take a course in copyediting.
In this case the relative clause is essential to the meaning of the sentence. If you embedded the clause in commas, then the meaning would change, suggesting that all lawyers need a course in copyediting.
Restrictive: The lawyer who has worked on this case for three years thinks that we have no chance of winning.
In this case the relative clause is essential to the meaning of the sentence. In other words, the sentence refers to only the lawyer who has worked on this case. The discussion is restricted to her.
Nonrestrictive: The lawyers, who have an office downtown, think that we have no chance of winning.
Because the location of the lawyer's office is superfluous to the gist of the sentence, it should be set off by commas.
Use Commas Before Nonrestrictive Adverbial Phrases or Clauses at the Ends of Sentences
At the end of your sentence, you need to be especially careful about where you place your commas. In particular, you need to question whether the modifying words are restrictive or nonrestrictive. For instance, suppose you received a memo from your writing instructor that said,
You should revise the essay, as I suggested.
You could assume that you were directed to revise the essay in any way you deem appropriate. However, if the instructor omitted the comma, then you would be receiving an entirely different message: revise the essay exactly as prescribed by the instructor.
Below are some additional sentences to give you a sense of how to determine whether your modifying words are restrictive or nonrestrictive:
Nonrestrictive: Reports indicate that a Turtle Excluder Device (TED) costs from $85 to $400, depending on the model.
Restrictive: Writers can change readers' outlooks on issues provided that they offer sufficient evidence.
In this case, a comma after issues could suggest that writers have numerous ways to change readers' opinions and that one of these methods is providing sufficient evidence. In contrast, the lack of a comma means that providing evidence is the one criterion writers need to follow.