Writers who aren't quite sure where to put their commas often use them to (a) join together sentence fragments in an effort to make a complete sentence, or (b) to break up a sentence that just, somehow, looks too long. Hence, one is likely to see a sentence like this:

One of the easiest ways to determine whether a comma is correct in these situations is to test whether you can swap it for a period. Try that with the sentence above:

See the problem? Let's look at some specifics...

Run-on Sentences

Let's start with commas and "run-on sentences." A "run-on" sentence is one in which several (usually short) phrases that are complete sentences in themselves are strung together to make an incomplete sentence. Such a sentence often ends up looking like this:

This is not, in fact, a sentence. How can you tell? Try substituting a period for the commas.

Notice that each sentence stands alone just perfectly. If you can substitute a period for a comma, then something is wrong with the comma. Here's an example in the reverse, using the sentence I just wrote: "If you can substitute a period for a comma, then something is wrong with the comma." Try the replacement here:

See? Neither of those two sentences stands alone (particularly the first). That's a good indication that you do need a comma!

Fortunately, there is a way to pull together a series of sentence fragments and join them into a complete sentence -- even a sentence like the one above. Start by inserting the word "and" where you originally tried to put your commas:

This is a grammatically correct sentence. It isn't a good one, but it's correct. It's also getting you closer to the sentence you were trying to write. The final step is to swap those "ands" for commas... with one huge exception:

This is the famous "serial comma," which is what you were aiming for in the first place. Note that you only actually had to make one change to your sentence to make it correct: insert that little "and" after the very last clause. That's all that was needed to change a bad sentence to a good one.

Incomplete Fragments

Another common comma misusage occurs when writers have the impression, "I just know a comma is needed here somewhere, but I'm not sure where, so, I'll, just put in a couple and, hope for the best."

Let's start with the "period/comma" substitution test and see what happens:

The first two clauses do, in fact, stand alone, so you can leave those commas alone. Technically, you could even leave "So" alone -- it's called an "interjection" (like "Aha!" and "Hey!"), and thus could stand alone if you absolutely wanted it to. In other words, you could leave a comma after it, or you could remove it. (The key there is to ask whether you actually want the reader to "pause" after reading "so"; if you do, leave it in; if you don't, leave it out.)

After that, well, it's pretty obvious that you don't have complete sentences. So start taking out periods and see what happens:

Whoops, no more commas! In fact, none were needed.

Now a quick digression: Some people object highly to starting a sentence with "but" or "and." I am not one of them. I don't think it should be done often, but it can be done -- as it does create a "complete thought." An alternative is to use "however," but this can create a more complicated sentence and isn't always what you want. "However" will nearly always need to be followed by a comma; "but" does not:

Dependent Clauses

One of most common uses of a comma is to set off a "dependent clause." A dependent clause is a part of a sentence that can't live on its own. It needs life-support. It generally does not have its own subject, or quite often even its own verb, like that clause I just inserted. It doesn't make sense without what went before.

Without commas, a dependent clause can create a run-on sentence that is very hard to understand. The commas tell you "hey, listen up, new thought or idea coming here!" For example, what if I wrote:

You could, probably, find your way through that sentence. But you'd probably have to read it more than once. You would probably be scratching your head a bit about the last bit - what clause was just inserted? What are you talking about?

Dependent clauses can be tricky to identify. One of the keys is to consider who depends upon whom. What part of the sentence can be removed without actually destroying the sentence, or its meaning?

In this example, I could simply say "It generally does not have its own subject." I could just leave it there, and you'd have a sentence. I could not say "Or quite often even its own verb" or "Like that clause I just inserted." Neither of those fragments stand alone, nor do they really make any sense on their own. What doesn't have its own verb? What clause was inserted, where? Without the first part of the sentence, those second and third clauses make no sense, which is why (ta-da!) they are dependent. They literally depend upon the rest of the sentence to make any sense at all.

The period-substitution test works to a certain degree here to help you identify if you need commas:

However, as you can see, the first sentence is actually a real sentence, so... the period test may not be sufficient. One of the primary problems with punctuating dependent clauses is that, when they occur in the middle of a sentence (like this one), writers often forget that they need to be punctuated at both ends. They need to be opened and closed with commas. (If they end a sentence, it's perfectly fine to end them with a period.)

A good way to identify a dependent clause that needs to be opened and closed with a comma is to try substituting, not a period, but a pair of parenthesis. A dependent clause can be set off with parentheses and still be correct:

Note that I did put a comma after the parenthesis, because in fact both the second and third clauses in that sentence were dependent. The only "standalone" clause was the first; it is the one that had a subject and a verb.

Once you identify a clause that can be set off with parentheses (which clearly must be opened and closed), it's much easier to determine where you need to put that second comma. Of course, a comma isn't your only option here; you could just go with the parentheses, or even dashes:

In this case it becomes understood that the third clause is dependent, as you never put a comma or any other punctuation directly before or after a dash.

However, most readers find commas easier to follow than parentheses and dashes. I admit that I use both, perhaps to excess, but keep in mind that each time you use a new bit of punctuation, you're causing the reader to interrupt his or her flow to absorb a new thought. Parentheses and dashes have their place, but should generally be used when you really, really want to call attention to a subordinate clause or secondary idea. Commas are such minor roadmarks to most readers that they don't cause a mental "bump in the road" when they are encountered (unless, of course, they are encountered in the wrong place).

More About Serial Commas

In the world of commas, nothing causes more controversy than the question of the serial comma: When to use it, and when to leave it out. Part of the problem is that rules change over time. Oldtimers like me were taught to use the so-called Oxford comma -- but this practice has been debated and/or discouraged by many newer style guides.

Simply put, the "Oxford comma" means using a comma at each stage of the serial comma. If I want to describe what I'm wearing to the ball, I might say:

But that brings us to the serial comma when used in a line of adjectives or adverbs in a description. This issue, too, has come under debate and has resulted in what many writers and grammarians consider to be an improper usage. What commonly happens, today, is for a writer to insert a comma after every item in a descriptive series.

Let's suppose, for example, that you are describing a young woman. You might write:

OK -- we would get the picture. But do you really need the comma between "blue-eyed" and "blonde"? Twenty years ago most writers would have said "no." Today, many people suggest putting that comma in -- apparently simply because it's easier than trying to figure out whether a comma is needed or not! This approach suggests that rather than thinking about the flow of a sentence, a writer is simply slapping in a comma at every possible juncture just to be on the safe side.

But keep in mind that the basic purpose of a comma is to introduce a slight pause into the flow of a sentence. If you took all the commas out of that sentence, it would be a bit breathless:

At each point where you place a comma, the reader gets a tiny pause, a chance to digest each bit of information before moving on to the next. But the last pause is generally thought to be unnecessary. Why? Because... If that were the only adjective in the sentence, no comma would be needed:

See? It wouldn't make much sense to write "Jennifer was a blue-eyed, blonde." It's not grammatically correct. And that's one reason why, traditionally, no comma is inserted between the final entry in a series of adjectives or adverbs and the noun (or verb) that they are modifying.

Another rationale that is given for not putting in that final comma is that the adjective or adverb closest to the word it is modifying should be the most significant. The general idea is that you could, if necessary, drop all the others. The word that is closest to the subject or verb is the word that you most want the reader to "connect" with that subject or verb. Often, it is an adjective that you can't do without -- one whose loss might change the meaning of your sentence entirely. For example:

If we removed the word "plastic," this sentence would read: "Koko the Clown had a red, bulbous nose." That means something very different! It's important for the reader to know that Koko is wearing a plastic nose -- if we omitted that, we'd worry about Koko's health. In a sense, the words "red" and "bulbous" actually modify "plastic" as much, or more, than they modify the word "nose."

Writers who object to putting that last comma in a series of adjectives point out that by doing so, the word "plastic" is demoted to the same status as the other adjectives -- i.e., it seems to become an adjective that could be removed from the sentence without harm. (Adjectives, after all, are like very tiny dependent clauses -- they may add a great deal, but you can live without them.) Since the word "plastic" can't be removed from the sentence without changing the meaning of the sentence, the comma does not belong there.

The Apposite Phrase

[Editor's Note: this section was so long that I have pulled it out and made it a standalone article, My Sister, Eileen... or My Sister Eileen? Understanding the Apposite Phrase.]

And That Brings Me to the Semicolon...

I know, I know: A semicolon is not a comma, even though it rather looks like one. However, when one is confused about where to put commas, one is often tempted to try to slap in a semicolon and hope it will solve the problem.

The best way to understand a semicolon is to realize that, unlike a comma, a semicolon actually is a substitute for a period. A semicolon is the "legal" way to join two standalone sentences. If you do it with a comma, you have an incomplete sentence. If you do it with a semicolon, you have a complete sentence. Could you have done it with that last sentence above? Try it:

No, actually, you don't. Why not? Try the period substitition:

In fact, you now have only one complete sentence (the second one). The first is incomplete -- so, it requires a comma. You have, in fact, a dependent clause at the beginning of your sentence (it starts with "if").

For a semicolon to be used, both clauses must stand alone:

Note that you can substitute a period for the semicolon. If you tried to substitute a comma, you'd have a run-on sentence.

Another use of the semicolon is to separate lists that include lots of commas, such as:

Without the semicolons, each of those names represents a similar object or item -- i.e., an item of "equal value. There would be nothing to inform the reader that "Detroit" and "Michigan" are not, in fact, simply two different cities. Keep in mind that there are many potential readers outside the US who may not know the names of US cities and states (and, unfortunately, there are quite a few folks within the US with the same problem). Another example:

Commas and Other Punctuation

A final source of "comma confusion" is where to place commas in conjunction with other forms of punctuation, such as quotes, parentheses, etc. Again, substitutions can often help here.

A mistake I often see is to place a comma before a dependent parenthetical clause, like this:

In this case, the parenthetical statement is a clause modifying the term "restaurant." You could, if you wished, remove it and still have a complete sentence. Part of the confusion here arises from the fact that if you so desired, you could remove the parentheses and use commas in their place:

I suspect that what may confuse the writer at this point is the sense that you are almost putting a comma after a comma substitute. By using the trusty "substitute a period and see what happens" technique, however, you can easily see where the comma has to go:

The first half of the sentence makes sense, which means that a comma must not have been needed here. The second half of the sentence doesn't make sense, which means something clearly is needed. The correct approach is to close off your entire "thought" before starting a new one, whether that thought has elements in parentheses, brackets, dashes, or commas:

But what about punctuation that belongs inside parentheses? It may look a bit silly to have "closing" punctuation both inside and outside parens, but it is necessary. The punctuation within parens should apply only to that specific phrase, while the punctuation outside the parens applies to your sentence as a whole:

Does it really matter? Does anyone care where your commas are? There are many who feel that such "nitpicking" over grammar is strictly for straitlaced purists. The bottom line, however, is that the task of writing is to communicate. We can never assume that our readers will, somehow, figure out what we mean; if we aren't sure, how can we hope that they will be? Punctuation is simply a tool that enables one to make oneself understood on the printed (or electronic) page.

Find Out More...

• Connect with Conjunctions, by Victoria Grossack
  
  
• Handling Gerunds and Their Relatives, by Dawn Copeman
  
  
• My Sister, Eileen... or My Sister Eileen? Understanding the Apposite Phrase, by Moira Allen
  
  
• Punctuating Dialogue, by Marg Gilks