How Do Commas Work?

We hear this question all the time: “How do commas work?” We even ask it ourselves, and we’re professional editors! So, we thought, “Let’s take a look at one of the most basic, most confusing pieces of punctuation ever: the comma.”

A comma in a hat holds out a hand to an interrobang wear party hats. The caption reads "I'm serial comma, but you can call me Oxford." People often ask "how do commas work?"
The serial comma at a punctuation party. (Medium)

At first glance, commas seem like they’re pretty simple to use. They separate ideas, clauses, and more, without introducing a full stop. Yet look at almost any two works of fiction, and you’ll quickly find that comma usage can vary a lot.

That leads to a lot of confusion among readers, writers, and even editors themselves. If you’ve been scratching your head and asking, “How do commas work anyway?”, this guide is for you.

What Is a Comma?

The first step to understanding comma usage is to know the role commas play in English grammar. Simply put, this piece of punctuation separates clauses and ideas within a full sentence. It represents a slight pause for the reader, but it is not a full stop, like a period, question mark, or exclamation point. You cannot end a sentence with a comma; if you try to do that, you’ll create a comma splice.

That makes the comma different from other types of punctuation, like the semicolon, the em dash, or even points of ellipsis. Any of those pieces of punctuation can be used to indicate the end of a thought and the start of another. A comma lets us keep going.

Separating Independent Clauses

The first use of a comma is to separate independent clauses within a sentence. These are usually two separate (complete) sentences joined by a conjunction, such as and or but.

I went to the store, and Sally went to visit her mother.

Here, we can see that we have two complete sentences joined by the conjunction and. If we dropped and, we would need to punctuate with a period:

I went to the store. Sally went to visit her mother.

Joining these two complete sentences by using a comma without a conjunction is a comma splice.

Setting Off Non-Essential Information in Subordinate Clauses

The next use of a comma makes things a bit trickier. Commas can be used like parentheses (/) and em dashes to set off non-essential information within a sentence. These are sometimes known as subordinate clauses. Effectually, they don’t matter to the sentence; you can pop them right out of the sentence or skip over them, and the sentence still represents a full and complete idea.

Sally went to visit her mother, who recently suffered a fall, over the weekend.

In this sentence, “who recently suffered a fall” is non-essential information. We don’t need to have it there to understand the rest of the sentence. If we were not to read it or to exclude it from the sentence altogether, the sentence would still make perfect sense. In effect, you can read these kind of clauses as “nice-to-have.” They offer a little bit more clarity or a little bit more information to the reader.

Sometimes, it can be difficult to figure out if information is truly integral to the sentence or not.

Sally has been very stressed because her mother is very ill and needs a good deal of extra care.

In theory, we could put a comma before because. “Sally has been very stressed” is a full and complete thought on its own. Yet the explanation that follows because might also feel like integral information, as it explains why Sally has been so strung out. If we put a comma before because, we might suggest that the reason Sally has been so stressed isn’t important. Of course, we can also argue that there should be a comma in front of because as it represents a conjunction joining two full and complete independent clauses.

As you can see: comma usage gets very sticky very fast!

Separating List Items

The third use for commas is separating list items. This is where we get into arguments over standard comma and serial (or Oxford) comma. Oxford comma says that every list item must have a comma separating it from the others, including one in front of and. Standard comma omits this final comma in front of the and.

A picture of two rhinocereses, George Washington, and Abraham Lincoln appears below a caption reading "With the Oxford Comma: We invited the rhinoceri, Washington, and Lincoln." The same text is repeated below, with the Oxford comma omitted. Only two rhinos appear, with Washington's and Lincoln's faces superimposed on them.
An example shows how the Oxford comma can eliminate ambiguity, but people still debate whether it’s strictly necessary. (Imagur)

Oxford comma is often preferred by book publishers (and editors) since it represents perfect clarity. Advocates of standard comma suggest that serial comma is only truly necessary in a few instances. However, ambiguity can arise from the use of standard comma, which is resolved through the use of serial comma. Therefore, we’d argue it’s preferable to use Oxford comma all the time. Why risk creating ambiguity when you can instead have perfect clarity?

How Do Commas Work: Two Schools of Thought

What we’ve outlined above are the basics of comma usage. As we noted, though, things can get complex fast, especially if you have lengthy sentences. As editors, we’ve noticed that there seem to be at least two “schools” of thought on comma usage.

The first school is the one we usually adhere to, and it’s the “grammatically correct” school. We assess commas for their grammatical use in a sentence. Most people tend to overuse commas, adding them where they are not strictly necessary. That, in turn, can create comma splices or improper subordinate clauses. It can even result in ambiguity when comma usage in a complex sentence accidentally creates a subordinate clause, which could imply some information is not integral to the sentence—even when it is.

Our work tends to use a lot fewer commas then. But that can result in sentences that are quite lengthy to read aloud, with very few pauses for breath. That’s where the other school of comma use comes in. This is the “emotional” or “naturalistic” use of commas.

Trying to use commas this way is more poetic, in a sense. Rather than looking strictly at what is grammatically necessary or correct, this school of thought adds commas wherever a pause feels necessary. Sometimes, this is used for dramatic effect. In other situations, it follows where the reader would naturally hesitate or pause, as if they were reading aloud or speaking.

This can be a lot friendlier for readers, even if it isn’t 100 percent “grammatical.” Unfortunately, it can result in overuse of commas and ambiguity in the writing.

A Couple of Common Comma Misconceptions

Whether you’re using the grammatically correct school or the naturalistic school, the serial comma or the standard comma, there are a few common pitfalls that crop up when people ask “how do you use commas?”

The first is the idea that you need a comma before every conjunction. This is just categorically untrue, although we see published works doing it all the time. We like to call this one “faux serial comma.”

Some people believe that because we use commas to join independent clauses with a coordinating conjunction, every conjunction needs a comma placed before it. Others believe that Oxford comma means putting a comma before every conjunction, even in a two-item list.

Nope! If you look at the Chicago Manual of Style, they specifically address the latter: Oxford comma does not mean putting a comma in front of the conjunction in a two-item list. We cringe when we see stuff like this:

I went to visit Jack, and Mabel.

That’s a two-item list; there’s no need for that comma in front of and! Similarly, writers (and editors) who believe that every conjunction needs a comma in front of it risk creating “sentence fragments” on the opposite side of the conjunction:

I spent one week of vacation in Florida, but the other at home.

“But the other at home” is not a complete sentence. Therefore, you don’t need a comma in front of but. In fact, putting it there creates an incomplete clause—which is now dependent (and shouldn’t be closed off like this, unless it’s non-essential information).

Lists with Three or More Coordinating Conjunctions

Another comma misconception arising from this mistaken comma-conjunction rule is the idea that a list constructed around and or or needs a comma in front of each conjunction. Again, we can look to CMoS to see this incorrect.

Natalie and Rachel and Linda went on a road trip.

No commas needed there! If there’s only one and, though, you should use either standard or serial comma.

Not Only/But Constructions

This is one of my favourite bugaboos: the “not only … but” construction. You cannot put a comma in front of the but here. It’s tempting, especially if you believe in the comma-conjunction rule, but it is technically incorrect.

Why? The not only requires whatever follows on the other side of the but to complete the sentence. Try it. Stick a period in front of but and take it out. Your not only feels like it’s hanging—not only this …

But … ? says your reader. They are waiting for you to complete that thought! Putting the comma in front of the but makes what follows a non-essential clause, meaning the reader could technically skip it. The use of not only precludes that, though. The rest of the sentence is absolutely necessary—which means you can’t put a comma there.

Still Wondering “How Do Commas Work”?

Don’t worry if you’re still not sure about where your commas do (and don’t) belong. As we noted, plenty of editors screw this up! (We have debates about it all the time too!)

If you’re looking for someone to help you wrangle commas and go over the finer points of “how do commas work,” give us a shout! We’ll worry about the commas and other grammatical nitpicks so you can focus on telling the story.