The comma tells the reader to pause, just as the blinking yellow light tells a driver to slow down and proceed with caution. Some writers can tell where a comma is needed by reading their prose aloud and inserting a comma where there seems to be a clear pause in the sentence. This may work much of the time if you read the sentence carefully and accurately. However, this procedure is not the most precise way to approach comma usage. Below are four general ways to use commas with a reasonable degree of certainty.

1) Between Items in a Series

When you are listing three or more items in a sentence, simply place a comma between each member of the list. Here are two examples:

Mr. Sanchez used the money that he won from the sweepstakes to buy a house, a car, and a small yacht.

We will purchase the stock if the price is lowered to $30 per share, if we are allowed to buy a block of over 10,000 shares, and if we receive a guarantee that no new shares will be created in the next fiscal year.

The commas above clearly mark where one member of the list leaves off and the next one begins. There is no mystery in how to use the comma in these kinds of sentences. What is often unclear, however, is whether to include the comma between the last and second-to-last items in a list. In the past, it was considered improper to omit the final comma in a series, but modern writers believe that the conjunction (and, but, or) does the same thing as a comma: it marks the place between two items in the set. These writers have argued that a sentence is more economical without an unneeded comma. As a result, you now have the option to choose whether to include the final comma.

Nevertheless, many people still follow the old rule and expect to see the final comma. Also, if your list is rather complex, omitting the comma may confuse the reader about where the second-to-last item leaves off and the last begins. In this case, of course, you would want to include the comma in order to avoid confusion. Perhaps it is best to get into the habit of always using the comma between the last two items in order to avoid all controversy. You do, however, have the option to omit it.

2) Between Two Sentences

You’ll remember that a semicolon is used to connect two sentences. However, more often we glue two sentences together with a comma and conjunction (such as and or but). In fact, if you examine a document you have written recently, you are likely to find many such sentences; they’re so common that you don’t even realize you are writing them. When you do put two sentences together with a conjunction, you must also include a comma. That is, the conjunction and comma are equivalent to a semicolon when you’re connecting sentences. Here are three examples:

The Suncom Corporation has just acquired the OILCO company, and it has agreed to sell OILCO’s oil-drilling rights in Texas as soon as possible.

I knew that the price of IBM stock would increase after it entered the home computer market, but I had no idea that the price would skyrocket.

I first conducted a thorough audit of the company, and I then interviewed the manager to try to determine how much money was missing.

Each sentence above is made up of two sentences glued together with a comma and conjunction. For example, the first sentence is made up of the following:

The Suncom Corporation has just acquired the OILCO company.

It has agreed to sell OILCO’s oil-drilling rights in Texas as soon as possible.

All you need remember is this: when you’re connecting two sentences with a conjunction, you must also include a comma because the conjunction and comma work together as a team. Perhaps this diagram will help you remember:

Often you may use a conjunction but not have a complete sentence on both sides of it. In this case you do not need a comma. For example, you could easily rewrite the above sentences so that one part of each sentence is not a full sentence:

The Suncom Corporation has just acquired the OILCO company and has agreed to sell OILCO’s oil-drilling rights in Texas as soon as possible.

I knew the price of IBM stock would increase after it entered the home computer market but had no idea that the price would skyrocket.

I first conducted a thorough audit of the company and then interviewed the manager to try to determine how much money was missing.

Because in the above examples you do not have full sentences on both sites of the conjunction, there’s no need to include a comma.

One last bit of advice: if your sentence is very short (perhaps 5 to 10 words), you do have the option of omitting the comma if you wish. You have this option because your reader can usually understand a short sentence more readily than a long one, and therefore you would not need a comma for readability. Here is an example:

Mr. Santana is old and he is wise.

This sentence is so short that you may omit the comma. Remember, punctuation is meant to help the writer and the reader, not to make their jobs more difficult. That’s why you may opt to omit the comma between these two short sentences.

3) To Attach Words to the Front or Back of Your Sentence

Most of the sentences we compose really consist of a short core sentence with many details added to that core sentence. Frequently, we add information to sentences by attaching one or more words to the front or back of the core sentence. You don’t need to memorize seven or eight rules naming each of the different structures you can add to your sentence. Instead, remember that when you add information to the front or back of a sentence, you will want to alert your want readers in order to help them clearly understand your message. Here are four examples:

Certainly, Joan is a successful salesperson.

Although she flunked chemistry and barely passed math, Joan is a good student.

In order to help save the company from bankruptcy, we sold shares in the company at discount prices.

Joan is a good student, although she flunked chemistry and barely passed math.

If you examine the sentences above, you will see where the writer has attached words to the front or back of each core sentence. Even when you add one word, such as certainly in the first example, you want your reader to know where the real sentence begins. This is why you place the comma there. If you read the sentences carefully, you’ll also notice a natural pause where the comma is situated.

4) On Both Sides of a Nonessential Component

The three uses of the comma just discussed are quite easy. You should be able to tell when those commas are needed or not. When you are proofreading your own prose, it will be clear to you whether you have a list of items or not, whether you’re attaching two sentences with a conjunction or not, and whether you are tacking words onto the front or back of your sentence or not. The fourth use of the comma, however, is a little more complex because you must make a judgment call. Nevertheless, even this fourth way to use a comma is relatively simple.

Often, you will insert a group of words into the middle of a sentence. Sometimes this group of words will need to be set off by commas from the rest of the sentence, and sometimes you will not need commas. In order to tell whether you need commas, you must make a judgment about whether the added words are essential to the meaning of the sentence or whether they simply provide extra detail. Let’s use an analogy to illustrate this concept. The modern stereo system is what we call a component system in that it is made up of many different components: an amplifier, receiver, CD player, tape deck, and various speakers. With most stereo systems, you have the option of removing certain components and adding new ones. For example, you may decide to unplug your tape deck but retain the CD player. The various components are optional or nonessential to the system itself. Similarly, you often add or delete components from your sentences. If a component (a group of words in this case) is added to a sentence but does not affect the meaning of the sentence when it is removed, then that component is not essential.

In order to tell a reader that a group of words is a nonessential component, you place commas in front and in back of the group of words. However, if omitting the group of words would drastically change the meaning of the sentence, then those words are not a component; rather, they are essential to the meaning of the sentence. In that case, you would not want to put commas on either side of the component so that the reader knows that those words are absolutely important to the meaning of the sentence. For example, look carefully at the following sentences:

Ms. Johnson, who is the company president, will present the award at our annual dinner.

Banks which hold over a billion dollars in assets are rare.

In the first sentence, the information about Johnson being the company president has no bearing on the main idea of the sentence: that she will present the awards at the annual dinner. Since this information is added or extra, we let the reader know it is an interchangeable component (like the stereo’s tape deck) by placing commas on either side of it. In contrast, the second sentence contains information that is absolutely essential to the meaning of the sentence: “which hold over a billion dollars in assets.” If you were to place commas around these words, you would be erroneously telling the reader that the words constitute a nonessential component. For example, look at this sentence:

Banks, which hold over a billion dollars in assets, are rare.

This sentence tells the reader that the main idea is that “banks are rare.” Certainly, banks are far from rare, but by enclosing the information in commas you have said that they are.

Although we’ve spent some time discussing this use of the comma, it isn’t very difficult to master. Simply remember that when you are proofreading you should check your sentences for essential and nonessential components.