These little demons compound and trivialize the nightmares of many a professor after an evening of reading student papers. A sure way to irritate educated readers of your work is to give them an overabundance of opportunities to address your comma problems. It is easy but dangerous to take the attitude that Sally once did in a Peanuts comic strip, asking Charlie Brown to correct her essay by showing her “where to sprinkle in the little curvy marks.”

You have probably heard the common tips on using commas: “Use one wherever you would naturally use a pause,” or “Read your work aloud, and whenever you feel yourself pausing, put in a comma.” These techniques help to a degree, but our ears tend to trick us and we need other avenues of attack. However, it seems impossible to remember or apply the 17 or so grammatical explanations of comma usage that you were probably introduced to way back in 8th grade. (For example: “Use commas to set off independent clauses joined by the common coordinating conjunctions. . . . Put a comma before the coordinating conjunction in a series.”) Perhaps the best and most instructive way, then, for us to approach the comma is to remember its fundamental function: it is a separator. Knowing this, it is useful to determine what sorts of things generally require separation. In sum, commas are used to separate complete ideas, descriptive phrases, and adjacent items, and before and after most transition words.

Comma Rules

Complete ideas need to be separated by a comma because, by definition, they could be grammatically autonomous, but the writer is choosing to link them. Complete ideas are potentially whole sentences that the writer chooses to link with a conjunction such as “and” or “but.”

Digital recordings made it possible to measure the nuclear magnetic signal at any depth, and this allowed for a precise reading to be taken at every six inches.

Note how the second half of this sentence contains both a subject (“this”) and a verb (“allowed”), indicating that a second complete idea is presented, and thus a comma is required.

Descriptive phrases often need to be separated from the things that they describe in order to clarify that the descriptive phrases are subordinate (i.e., they relate to the sentence context, but are less responsible for creating meaning than the sentence’s subject and verb). Descriptive phrases tend to come at the very beginning of a sentence, right after the subject of a sentence, or at the very end of a sentence.

Near the end of the eighteenth century, James Hutton introduced a point of view that radically changed scientists’ thinking about geologic processes.

James Lovelock, who first measured CFCs globally, said in 1973 that CFCs constituted no conceivable hazard.

All of the major industrialized nations approved, making the possibility a reality.

In each of these cases, note how the material separated by the comma (e.g., “making the possibility a reality”) is subordinate—i.e., it carries context in the sentence, but the primary sentence meaning is still derived from the subject and verb. In each example, the phrase separated by the comma could be deleted from the sentence without destroying the sentence’s basic meaning.

Adjacent items are words or phrases that have some sort of parallel relationship, yet are different from each other in meaning. Adjacent items are separated so that the reader can consider each item individually.

Weathering may extend only a few centimeters beyond the zone in fresh granite, metamorphic rocks, sandstone, shale, and other rocks.

The river caught fire on July 4, 1968, in Cleveland, Ohio.

This approach increases homogeneity, reduces the heating time, and creates a more uniform microstructure.

In the first sentence, the commas are important because each item presented is distinctly different from its adjacent item. In the second example, the dates (July 4, 1968) and places (Cleveland, Ohio) are juxtaposed, and commas are needed because the juxtaposed items are clearly different from each other. In the third example, the three phrases, all beginning with different verbs, are parallel, and the commas work with the verbs to demonstrate that “This approach” has three distinctly different impacts.

Finally, transition words add new viewpoints to your material; commas before and after transition words help to separate them from the sentence ideas they are describing. Transition words tend to appear at the beginning of or in the middle of a sentence, and, by definition, the transition word creates context that links to the preceding sentence. Typical transition words that require commas before and after them include however, thus, therefore, also, and nevertheless.

Therefore, the natural gas industry can only be understood fully through an analysis of these recent political changes.

The lead precursor was prepared, however, by reacting pure lead acetate with sodium isopropoxide.

Using a Comma Before “And”

It is true that commas are sometimes optional, depending on sentence meaning and the writer’s taste, and many writers choose not to put a comma before the “and” in a series (also known as the “serial comma”) involving a parallel list of words. For example, some would write the sentence “I am industrious, resourceful and loyal,” using no comma before the “and.” This practice is fine as long as you are consistent in applying it. However, I, and the grammar handbooks I consult, recommend a comma even in these circumstances, because—even in the example provided—there is a slight pitch and meaning change between the terms “resourceful” and “loyal.”

Most importantly, if the “and” is part of a series of three or more phrases (groups of words) as opposed to single words, you should use a comma before the “and” to keep the reader from confusing the phrases with each other.

Medical histories taken about each subject included smoking history, frequency of exercise, current height and weight, and recent weight gain.

By always using a comma before the “and” in any series of three or more, you honor the distinctions between each of the separated items, and you avoid any potential reader confusion. The bottom line is this: When you use a comma before the “and” in a series of three or more items or phrases, you are always correct.

That noted, be aware that some professors and many journals will not favor the use of the comma before an “and” in a series (for the journals, it is literally cheaper to print fewer commas).

Comma Overuse

Perhaps the best way to troubleshoot your particular comma problems, especially if they are serious, is to identify and understand the patterns of your errors. We tend to make the same mistakes over and over again; in fact, many writers develop the unfortunate habit of automatically putting commas into slots such as these:

• between the subject and verb of a sentence
• after any number
• before any preposition
• before or after any conjunction

Thus, incorrect sentences such as these appear in papers:

The bushings, must be adjusted weekly, to ensure that the motor is not damaged.

Many botanists still do not fully appreciate these findings even after 22 years, following the publication of the discovery paper.

Other manufactured chemicals that also contain bromine are superior for extinguishing fires in situations where people, and electronics are likely to be present.

The price of platinum will rise, or fall depending on several distinct factors.

If the commas above look fine to you, then you may be in the habit of using commas incorrectly, and you will need to attack your specific habits, perhaps even in a routine, repetitive fashion, in order to break yourself of them. Similarly, it is common for someone to have to look up the same tricky word dozens of times before committing its proper spelling to memory. As with spelling, commas (or the absence of commas) must be repeatedly challenged in your writing. As you perfect your comma usage you are also recognizing and reevaluating your sentence patterns, and the rewards are numerous. There is no foolproof or easy way to exorcise all of your comma demons, but reminding yourself of the comma’s basic function as a separator and justifying the separation of elements whenever you use the comma is a good beginning. I often recommend to students with comma problems that they re-read their work one last time, just focusing on their comma use, before turning in a paper as a final version. In the end, you simply must make a habit of reading, writing, and revising with comma correctness in mind, and remember that commas have much to do with sentence wording, which is always in the control of the writer.

To demonstrate this last point, Lewis Thomas, a clever essayist as well as a physician and poet, shows us how to use commas effectively—as well as how to word a long sentence so that commas are not overused—in this excerpt from “Notes on Punctuation”:

The commas are the most useful and usable of all the stops. It is highly important to put them in place as you go along. If you try to come back after doing a paragraph and stick them in the various spots that tempt you you will discover that they tend to swarm like minnows into all sorts of crevices whose existence you hadn’t realized and before you know it the whole long sentence becomes immobilized and lashed up squirming in commas. Better to use them sparingly, and with affection, precisely when the need for one arises, nicely, by itself.