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Objective
In this lesson,
you'll learn why commas are necessary in writing and the most common
principles of comma usage.
The Necessity
of Commas
Commas are signals
that help readers understand the meaning writers create when they
compose sentences. These signals tell readers, for instance, which
words need to be separated from each other or which words are not
essential to the basic grammar of a sentence. In short, commas help
readers make sense of writing because they provide pieces of information
necessary for complete comprehension of a writer's ideas.
An example sentence
highlights the necessity of commas:
For Susan
Mary was the perfect mentor.
As it stands,
this sentence is difficult to understand. Is the writer trying to
communicate something about a person named "Susan Mary," and has this
writer simply left out some important words? Or, is this writer trying
to communicate something about two separate people, "Susan" and "Mary"?
When a comma
is added, however, the meaning of this sentence is perfectly clear:
For Susan,
Mary was the perfect mentor.
With the added
comma, anyone who reads this sentence understands that the writer
is talking about two separate people who've had a professional mentoring
relationship, whereas without the comma, the meaning of this sentence
is ambiguous.
Common Principles
of Comma Usage
There are many
principles guiding comma usage, and yet there are a few that are used
most frequently. Understanding these basic principles and being able
to apply them in your own writing will help ensure that readers understand
what you're trying to communicate.
(1) Use a comma
to separate introductory elements at the beginning of a sentence
from the main, independent clause:
Introductory
element, main clause.
Words, phrases,
or clauses that are placed before the main independent clause of a
sentence are known as "introductory elements." Most often, these introductory
elements tell when, where, why, or how the action expressed by the
predicate occurred. A comma is needed at the end of an introductory
element to signal to readers that the introduction has ended and the
main independent clause will follow:
In the morning,
I like to drink coffee.
On the sunny
porch, she placed her most ailing plants.
Because we
were thrilled about winning the daily lottery, we jumped up and down.
Like screeching
parrots, my friends and I sang at the top of our lungs.
Note:
If an introductory element is short and there's no possibility of
misreading it, then the comma can be omitted:
Later she
went to the store.
However, it is
usually best to get in the habit of placing a comma after every introductory
element.
Later, she
went to the store.
(2) Use commas
and a coordinating conjunction to separate three or more items
(words, phrases, or clauses) listed in a series:
Main clause item,
item, coordinating conjunction item.
Note:
See Lesson 5 for an explanation of coordinating conjunctions.
A comma is needed
between brief items in a list to signal to readers when one item ends
and another one begins:
I tie-dyed
my t-shirt various shades of yellow, orange, and red.
They wanted
to go downtown, see a movie, and then eat dinner.
My parents
hope that I'd go to college, finish my degree, and get a good job.
Note:
According to some writing handbooks, the comma before the coordinating
conjunction is considered optional. However, this principle of comma
usage applies to journalistic writing, such as in newspapers and magazines;
in academic writing, the comma should always be included before the
coordinating conjunction.
(3) Use a comma
to separate coordinate adjectives:
adjective , adjective
noun
Coordinate adjectives
are two or more descriptive words that provide additional information
about a noun or pronoun. If these adjectives are placed next to each
other, commas must separate them:
The hot, smoky
room absolutely took my breath away.
She was an
enthusiastic, energetic moderator.
Although she
wanted to go swimming, she settled for splashing around in a pool
of murky, oil-spattered water.
(4) Use a pair
of commas to separate nonessential elements (phrases and clauses)
from the main clause:
Part of main
clause, nonessential element, part of main clause.
Phrases and clauses
that are not necessary for an independent clause to be considered
a complete sentence are known as "nonessential" (or "restrictive")
elements. Generally, these nonessential elements provide additional
description of nouns and pronouns.
Carol, a registered
nurse, supervised the wellness screening.
The horses,
freshly brushed, scampered about the riding ring.
I wanted
to tell my brother, my confidante for years, what happened.
In these sentences,
the phrases enclosed in commas provide additional information about
the nouns or pronouns they're describing; and yet, these phrases are
not essential for each independent clause to be a grammatically complete
sentence. Consequently, the commas are needed to signal the nonessential
nature of these phrases.
Sometimes, a
particular word can suggest whether a phrase or clause is nonessential
or essential. Usually, the word which is used to begin a phrase
or clause not essential to understanding the precise meaning of a
sentence, while the word that is used with a phrase or clause
absolutely essential to understanding the precise meaning of a sentence:
The phone,
which had a shrill tone, rang incessantly.
(nonessential phrase introduced by "which," so commas are needed)
The phone
that connected her home office to her company's headquarters rang
incessantly.
(essential phrase introduced by "that," so no commas are needed)
In the first
sentence, the implication is that there is a single phone in the house;
therefore, describing its tone isn't an essential part of understanding
the exact meaning of the sentence. As a result, the phrase "which
had a shrill tone" is enclosed in commas. However, in the second
sentence, the implication is that there are multiple phones in the
house, and that the only one ringing constantly was the one connecting
the home office to the company's building. In this case, the phrase
"that connected her home office to her company's headquarters"
is essential to understanding the exact meaning of the sentence, and
therefore it's not enclosed in commas.
Summary
Commas are signals
used to help clarify the meaning of writing. When placed according
to common principles of usage, commas can help readers understand
a piece of writing. As writers, we need to know how to use commas
appropriately to help readers clearly comprehend the message we're
communicating.
  
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