This rule also applies to exclamations…

Alas, he was not there.

Damn, that purse-snatcher was fast.

…and direct address:

I’m sorry, ma’am, I lost him.

Officer, I saw the whole thing.

 

Parenthetical/Non-essential Elements

Phrases or clauses that can be omitted from a sentence without changing its basic meaning should be set off by commas.

The purse-snatcher, a small man, ran past me.

The purse-snatcher, who wore a ski cap, ran past me.

Here the information about the purse-snatcher’s size and haberdashery is secondary to the main point of the sentence: he ran that way.

Don’t be misled into setting off a restrictive clause with commas. A restrictive clause, usually introduced by that or who, gives essential information, as opposed to a non-restrictive clause, which is introduced by which. That clauses don’t get commas, but which clauses do; in modern writing the distinction between which and that has been blurred, leading to occasional confusion over where to put the commas.

The purse which was from DKNY was red.

Although the above sentence is correct in common usage, it leads to confusion over meaning; it could be interpreted as meaning either of the following:

The purse, which was from DKNY, was red. (The purse is important; its DKNY-ness

is only mildly interesting)

The purse that was from DKNY was red. (The DKNY purse, rather than any other purse,

is important)

To avoid this kind of confusion, always consider whether the information contained in the phrase is essential.

 

Coordinate Adjectives

Two adjectives that modify one noun should be separated by commas if they both have the same descriptive force:

The silly, frightened purse-snatcher dropped the purse with all its contents.

However, adjectives should not be separated by a comma if they are not coordinate; that is, if they do not have the same importance as describers:

I returned the purse to the little old lady from whom it was stolen.

NOT: I returned the purse to the little, old lady from whom it was stolen.

 

In this example, little and old are not coordinate adjectives because old has more force than little: old lady runs together as a complete idea. The quick way to determine if two adjectives are coordinate is to put an and between them; if the phrase still sounds right, then you can replace the and with a comma. The silly and frightened purse-snatcher makes sense, so silly, frightened purse-snatcher is correct; by comparison, little and old lady doesn’t really mean the same thing as little old lady.

 

Complementary or Antithetical Elements

Two phrases that both refer to one word or share a function in a sentence should be separated by commas. This construction usually expresses contrast:

I found the purse, not the purse-snatcher.

The older, but still beautiful, owner of the purse was quite grateful.

 

Series and Lists

In a series consisting of three or more elements, the first two elements should always be separated by commas if they are not separated by conjunctions:

The little old lady, the cop, and I all went down to the precinct.

NOT: The little old lady, and the cop, and I all went down to the precinct.

Sometimes, writers will drop the comma before the final conjunction in a series:

The little old lady, the cop and I all went down to the precinct.

This final comma is called a serial or Oxford comma, because it used to be standard in British English but not in American English. Using the Oxford comma is optional; however, if you have a particularly complicated series consisting of many elements, this comma can do a lot to eliminate confusion:

The little old lady whose red DKNY purse was stolen, the cop who was first on the scene

and who didn’t do a damn thing to help me catch the purse-snatcher, and I all

went down to the precinct.

NOT: The little old lady whose red DKNY purse was stolen, the cop who was first on the

scene and who didn’t do a damn thing to help me catch the purse-snatcher and I

all went down to the precinct.

 

Elliptical Constructions

Sometimes commas are used to indicate omitted material in a sentence that can be understood from context:

The little old lady’s statement took forty-five minutes; mine, four hours.

However, if you don’t need these commas for clarity, don’t use them.

 

Quotations

Direct quotations of complete thoughts are often introduced by commas:

The cop asked me, “What did you think you were doing, chasing that guy?”

I responded, “Your job, since you weren’t doing it.”

But when quotations are incorporated into sentences, they are punctuated according to the needs of the sentence:

Telling the cop “Your job, since you weren’t doing it” might not have been the best idea.

In more formal prose, quotations are often introduced by colons rather than commas.

 

Questions

When a sentence includes a direct question but does not solely consist of that question, the question is usually set off from the rest of the sentence using commas:

I asked myself, how do I get myself in these situations?

The cop wanted to know, who did I think I was?

But when these questions come at the beginning of a sentence, the question mark does the comma’s job:

When are they going to let me go home? I wondered.

What was he doing on that street corner? the cop pondered.

If the question is indirect, it doesn’t need commas:

I found myself wondering where that purse-snatcher was now.

NOT: I found myself wondering, where that purse-snatcher was now.

 

Titles, Addresses, and Dates

Commas should be used to set off titles or positions following a name:

Captain Bitko, head of the theft department, finally came to the interview room.

Commas should also come between the individual elements of an address:

I told him I lived at 221B Baker Street, London, England.

Commas usually separate the date and the year if the day of the month is involved, but not if only the month and year are mentioned:

I had arrived in Durham in May 2004.

I had arrived in Durham on May 21, 2004.

 

But in the style preferred by the University of Chicago (also known as international or military format) there are no commas:

I had arrived in Durham on 21 May 2004.

 

A Final Thought…

Sometimes, even after you apply all these rules, you still find yourself with a tangled sentence that needs some commas to straighten itself out. On the other hand, sometimes your prose becomes confused because you have too many commas. Usually, writers use either too many or too few commas consistently; if you find out what kind of writer you are, you can decide whether you ought to look for commas to delete, or look for sentences that need more punctuation.