Commas Rule! Common Comma Rules and Tips

Hayley Salo, Writing Consultant

There are a ton of guides to comma rules, so I won’t spend this entire blog post rewording what those rules are. Instead, I’d like to take the time to go through the most common mistakes and discuss ways to identify, correct, and avoid them. This will require a bit of rule discussion, but bear with me.

Connecting Complete Sentences

There are multiple ways to connect complete sentences, and commas are certainly one of them! However, this is also where most comma mistakes are made, including comma splices and many run-on sentences. So, what’s the deal? Why is this part so hard?

Well, we tend to talk differently than we write; the pauses we make while speaking often do not match the pauses we grammatically require. Although this goes against the age-old advice of “read your work out loud,” it’s true! Reading your work out loud is a great way to catch comma mistakes that you are already familiar with, but it’s less effective at helping you catch the really tricky mistakes, including comma splices.

For instance, if we read the following sentence out loud, it sounds pretty normal:

Sally went to the store, and she bought an apple.

But so does this sentence:

Sally went to the store and she bought an apple.

The comma here is very hard to hear. Since the “and” tells us how the sentences are connected, it’s easy to assume that the “and” implies the pause, too. However, we need both the comma and the “and” for this sentence to be grammatically correct (Sally went to the store, and she bought an apple).

Since it’s hard to spot the missing comma while just reading an essay, it’s helpful to proofread one sentence at a time. Take the time to divide long sentences into two or more shorter sentences. Then, proof the punctuation by referring to the rules and recombining the sentence. The example above can be divided into two completely separate sentences:

Sally went to the store. She bought an apple.

Once we have the sentence divided, we can check the comma rules for how to connect complete sentences. We would see that we need both a comma and a connecting word, so we would know to combine the sentences into the following:

Sally went to the store, and she bought an apple.

This process of dividing sentences will become very important once the sentences get more complex.

Lists

Lists can be surprisingly difficult to proofread, and there are even two correct ways to punctuate the same list! Long, complex lists can be challenging because it’s hard for writers and readers alike to separate all of the ideas. As a result, dividing lists into shorter, simpler sentences is a great way to proofread. Let’s look at a simple first example before getting into a tougher one.

Correct: I like to walk, hike, and swim.

Correct: I like to walk, hike and swim.

Incorrect: I like to walk, hike, swim.

The first correct version uses the “Oxford comma,” which is just the optional comma before the “and.” The second correct version does not use that optional comma. The key here is that “and” is always required between the second to last and last list items, no matter how long or complex the list is.

As I mentioned earlier, we can divide lists into shorter, simpler sentences:

Correct: I like to walk. I like to hike. I like to swim.

This division makes it easier to see that “walk,” “hike,” and “swim” are all things I like to do. As a result, they are all part of the same list. But what happens when lists get more . . . listy?

Correct: I like dogs and cats, cake and cookies, and coffee and tea.

Correct: I like dogs and cats, cake and cookies and coffee and tea.

Incorrect: I like dogs and cats, cake and cookies, coffee and tea.

In the above examples, we have lists within a list. The primary list is things I like. We can see this more easily by dividing the sentence:

I like dogs. I like cats. I like cake. I like cookies. I like coffee. I like tea.

Very few people want to read that many short sentences. However, they are equally unlikely to want to read this long of a list:

I like dogs, cats, cake, cookies, coffee, and tea.

In long lists like these ones, readers are likely to remember only the first or last list items and tune out the middle ones. This is where our original example, with more than one “and,” comes in. We can simplify that list to a lesser extent:

I like dogs and cats. I like cake and cookies. I like coffee and tea.

Now it’s easier to see the categories of things I like: animals, food, and drinks. So, what we really have is a list of three things I like, but within that list, there are two-item lists arranged by category. Sounds pretty abstract, right? But it’s so much easier to see when it’s divided up like we did above. Once we know that we have lists within a list, it’s easier to know that we need an “and” between items in each two-item list, a comma between each category, and a final “and” between the second to last and last two-item list:

Correct: I like dogs and cats, cake and cookies, and coffee and tea.

Correct: I like dogs and cats, cake and cookies and coffee and tea.

Feel free to use or omit the Oxford comma.

Bottom line: lists can feel like a theoretical wormhole. Break them down into their smallest components and then carefully, deliberately, put them back together. We often write over-complicated lists in first drafts, so it’s up to us as later proofreaders to come back and fix them.

Optional Information

Generally speaking, optional information is separated from the rest of the sentence by commas. However, sometimes it’s hard to tell what is optional and what isn’t, which makes proofreading for comma mistakes very difficult. The trick here is to determine if the sentence’s core meaning changes when the information is removed. Let’s look at a few examples:

The dictionary, which is blue, is on the table.

“Which is blue” is grammatically optional; we can take that part of the sentence out and still have a complete sentence with the same meaning:

The book is on the table.

Nothing fundamental about this sentence has changed. We are still talking about the same book and the same table.

However, some writers and readers may not consider that information optional in certain situations. Take the following situation, which takes place in a library with a ton of books scattered around the floor, shelves, and table:

Sally: I need you to look up a word for me.

James: Where do I find the dictionary?

Sally: The dictionary, which is blue, is on the table.

James certainly does need to know that the dictionary is both blue and on the table in order to efficiently find the dictionary. However, it still isn’t the main point of the sentence. James could still find the same dictionary without that specification.

Commas shouldn’t be used when the “that,” “which,” etc. section really does affect the meaning of the sentence:

The dictionary that I own is in bad shape.

This sentence is talking about the physical appearance of my personal dictionary. If we take out “that I own,” we get something very different:

The dictionary is in bad shape.

This sentence implies that someone needs to do some serious editing and save our dictionaries! Not the same as our first sentence at all, so skip the commas.

Summary

When working with commas, try these tips:

  1. Read the sentence out loud to get a general feeling for what it is saying and how it is saying it.
  2. Divide the sentence into shorter, simpler sentences.
  3. Look up the rule for the kind of sentence you’re working on.
  4. Apply the rule and carefully recombine the sentence.
  5. Proofread similar sentences in the paper one after another to practice the rules and methods.

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