Comma Errors

If anybody ever told you that you should use a comma wherever you pause as you read a sentence out loud – as somebody very likely has at some point or other – they may have thought they had your best interests in mind.  But they weren't giving you good advice.  

It's possible they themselves may not have fully understood how commas work, but they may also simply have figured it would be easier for you to think about it this way.  And to be fair, there is some truth to the rule of thumb about pauses, which is kind of a ballpark thing:  generally speaking, it’s probably right at least as often as it isn’t.  

But we think you should shoot for better than that.  

The truth is that the commas people really have trouble with are signals writers give readers about the structure of sentences – about how they should understand the pieces of the sentence and how they fit together.  Commas help us process the major chunks of sentences and the relations between them as we read.  

What we’ve done here, then, is try to describe those signals, to make clear what given commas are supposed to suggest about the sentences they appear in.  

But we'll also try to detail some simpler uses of commas -- ones that are less a matter of sentence structure and more a matter of what's customary.  You're probably not confused about those commas, but we'll point out some of the most common just in case.

Structural Commas

We identify six especially significant structural reasons for commas:

Rule 1:  AFTER INTRODUCTORY PHRASES AND CLAUSES

You should use a comma to mark the end of any significant introductory phrase or clause, so that the reader understands where the real sentence (or independent clause) begins, what the subject and verb really are.  Like this:

Just as he was calling, Olivia texted him from the bakery.

Some sentences, like the one above, will not only be easier to read with commas; it may create real confusion if you leave them out.  Imagine the sentence above without the comma:  

Just as he was calling Olivia texted him from the bakery.  

Until you got close to the end of the sentence, you probably thought its ideas was that he was calling Olivia.  That's why commas like these are so helpful.  It makes clear that Olivia is the sentence subject, not the object in an opening phrase or clause.  

Rule 2:  BETWEEN THE ELEMENTS IN ANY LIST

You should use commas between all items in a list.  Like this:

Ronelle loves borscht, peirogies, golumpki, and all other Polish food. 

This applies not only to objects and other nouns, like in the sentence above, but to other parts of speech as well.  So you'd do the same with verbs, for example:  

Justine selected, downloaded, and saved each file individually.  

Or even phrases of different sorts:

He talked about what Lincoln called government "of the people, by the people, and for the people."

A Related Fine Point:  You've probably heard about "Oxford commas."  The last comma before the ands in each of these lists is an "Oxford comma," also known as a "serial comma."  This comma is optional -- you can choose to use it or not.  Writers in some disciplines tend to use it more than others, and we generally recommend it, since sometimes elements in a list can have internal commas.  But as long as you either use them or leave them out consistently, it's up to you.

Rule 3:  BEFORE AND, BUT, & OR WHEN THEY CONNECT INDEPENDENT CLAUSES

You should use a comma anytime you connect two independent clauses with an and, a but, or an or.  So:

Jordan went to the park, and he ran seven laps around the reservoir.

But not here:

Jordan went to the park and ran seven laps around the reservoir.

Why a comma here in the first sentence but not the second?   Because in the first sentence, there are two paired subjects and verbs -- two independent clauses -- connected by and:  that is, Jordan went and he ran.  It's a compound sentence.  But the second sentence has two verbs that follow from the same subject:  Jordan went and ran.  It's a single indepent clause with two verbs -- Jordan did two things.

So you should use a comma before the conjunction (and, but, or, and a few others) in any compound sentence.  This makes what's coming clear for readers:  if they see a comma, they'll look for another subject and verb.

One note here:  many style guides suggest leaving these commas out in especially simple sentences, but we recommend using them even then.  Like this:  The sun went down, and darkness came.  But it's not something readers necessarily expect -- whether you choose to do it is up to you.

Rule 4:  AROUND CONJUNCTIVE ADVERBS (words that cue readers about connections between sentences)

You should use commas any time you add a conjunctive adverb to a sentence -- that is, a word or phrase suggesting how the sentence is connected to the previous one, like moreover, in fact, or for example.  Like this:

When he chose a car to buy, however, purchase price was his primary consideration. 

Conjunctive adverbs can come at the beginning, in the middle, or at the end of sentences, and you should use commas in all cases.  So in addition to the sentence above, you could also say:

However, when he chose a car to buy, purchase price was his primary consideration.

When he chose a car to buy, purchase price was his primary consideration, however.

Rule 5:  BETWEEN ADJECTIVES THAT DON’T ALL WORK TOGETHER ("coordinate" but not "cumulative" adjectives)

You should use a comma between adjectives that modify nouns independently.  So:

Ignoring sensible, frequently recommended precautions, the driver of the big green pickup truck continued passing on the right.

Why a comma between sensible and frequently recommended but not between big and green or green and pickup?  Aren't they all strings of adjectives describing the nouns (precautions and truck) that follow them?  

Sure. But notice that sensible and frequently recommended modify precautions kind of independently, separate from one another, while big, green and pickup modify truck together, as part of single picture.  That means sensible and frequently recommended are what some people call "coordinate" adjectives, and big, green, and pickup are what some people call "cumulative" adjectives.  

Here's the simplest way to think about this:  you could put an and between coordinate adjectives, which take commas, but not between cumulative adjectives, which don't.  Does it seem natural to say sensible and frequently recommended precautions?  Sure.  Then we should say sensible, frequently recommended precautions.  Does it seem natural to say the big and green and pickup truck?  Of course not.  Then no commas:  the big green pickup truck

So use commas between adjectives that do not work together and not between those that do work together.  Or more simply:  if you can put an and between them, use a comma; if not, then don't.

Rule 6:  AROUND NON-RESTRICTIVE ELEMENTS (that is, when they DON’T identify the nouns they modify)

You should use commas around phrases and clauses that describe but don't identify nouns.  So:

The tomato plant treated with fertilizer daily looked beautiful.  

The tomato plant, treated with fertilizer daily, looked beautiful.

This is kind of tricky.  Both these sentences are punctuated correctly.  But they mean something different.  

In the first, you might imagine you're in a greenhouse where somebody's been testing the effects of different amounts of fertilizer, so the phrase treated with fertilizer daily identifies which tomato plant you're talking about.  In the second, you're on somebody's patio admiring their tomato:  there's only one plant, and treated with fertilizer daily just adds some information, in this case describing why it looked beautiful. 

Remember that the issue here isn't whether the information is "extra" or not, as people sometimes put it.  It's about whether it serves to identify the noun it describes or not.  In the second sentence about the tomato plant, treated with fertilizer daily is really a vital part of the sentence, explaining why its assertion is true.  But it doesn't identify the tomato plant.

Some people use the language of "restrictive" and "non-restrictive" elements to describe this -- that is, whether an adjective "restricts" the identity of the noun it describes or not.

So use a comma for non-restrictive elements, when an adjective doesn't identify the noun it describes, but not for restrictive elements, when an adjective does identify the noun it describes. 

Here are a few other examples representing other common moments or structures in sentences where you'd use this sort of comma:

APPOSITIVES:   My brother, an experienced plumber, came to help.

PREPOSITIONAL PHRASES:  The short essay, with its provocative line about "remember[ing] the very day that I became colored," is perhaps Hurston's best known work.

PARTICIPIAL PHRASES:  Helen, rising from her seat uncomfortably, pretended to know nothing. 

Recognize, however, that these moments and structures don't necessarily require commas -- it all depends on whether the element identifies the noun it describes or not.  

So while we'd say Helen, rising from her seat uncomfortably, pretended to know nothing, we'd also say The woman rising from her seat uncomfortably pretended to know nothing.  In the second version of the sentence, we don't know the identity of the girl except through the phrase that describes her:  rising from her seat uncomfortably.  So no commas.  In fact, if we included the commas, readers would assume they knew what woman we were talking about!

Recognize too that we can identify a noun in a wholly subjective, even debatable way, and elements offering further description would still be non-restrictive and so require commas.  Of course, we'd need the commas in this sentence:  

Cake’s second album, Fashion Nugget, contains the band’s best-known work.  

But we'd need them in this one too, even if we liked some other record by the same band better:

Cake’s finest album, Fashion Nugget, contains the band’s best-known work.

All that matters is that we've identified it in some way.  If we don't identify it in any way, of course, no commas:  Cake’s album Fashion Nugget contains the band’s best-known work.

This is a subtle difference, sure.  But it's an important one, and being able to write sentences that manage it effectively shows your grasp of a significant nuance of written English.  

A Related Fine Point:  Most elements can be either restrictive or non-restrictive, but you may have noticed that clauses beginning with that and clauses beginning with which are handled differently.  That clauses are always restrictive and never take commas; which clauses are almost always non-restrictive and so take commas.  Here's an example:

The publishing company that first printed Allen Ginsberg's controversial Howl was founded in a bookstore in San Francisco's Chinatown in 1955.

City Lights Books, which first printed Allen Ginsberg's controversial Howl, was founded in a bookstore in San Francisco's Chinatown in 1955.

You couldn't switch the which and that in these two sentences, and you couldn't either use commas in the first or leave them out in the second.

Other Comma Uses

Here are some other places you'll see commas, less connected to structural issues in sentences -- and probably less confusing, too:

In place names:  Gallup, New Mexico

In dates:  March 4, 2011

In certain forms of quoting:  Tina said, "Forget it!"   Or:  According to Oscar Wilde, "No good deed goes unpunished."

With people's titles:  Dr. Mary Walker, M.D.    

When addressing a reader or listener directly:   I wonder, Tom, if you've thought carefully about this?

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