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Let’s talk about commas. Below are some guidelines for using them correctly.
Use a comma after all introductory elements.
Any element, regardless of length, coming before the main clause should be punctuated with a comma. The main clause is that core part of a sentence that makes it a complete sentence; that is, it expresses a complete thought. Below are some examples of how to use commas after introductory elements. (Introductory elements are italicized.)
Examples
When an atom acquires enough energy to leave its orbit, the atom is positively charged.
As for the energy required to produce plastic automobile parts, the auto makers view the additional cost as justified by the savings in petroleum by a lighter car during its lifetime.
Because the high-pressure turbopumps rotate at speeds of 30,000 rpm, the weight distribution on the turbine blades must be balanced with great accuracy.
Because there is no belt of doldrums in the Atlantic south of the equator, hurricanes do not usually occur there.
Between 40 and 50 degrees west and just south of 10 degrees north in the western end of the doldrums belt, calms do occur with frequency, and hurricanes often originate there.
Even single words and short introductory phrases require commas.
Examples
In 1831, Michael Faraday discovered that if a magnet was moved in the vicinity of a coil, a current could be induced in the coil.
Using this concept, Faraday arrived at a relation between the changing flux and the induced electromagnetic field.
Today, the computer consortium is announcing its new chip.
Double-check commas between the parts of a sentence.
A single comma should never break the flow of the subject, verb, and object or complement of a sentence. Below are some examples. (The bracketed commas indicate where erroneous commas are typically placed.)
Examples
The discovery that moving a magnet within a coil could produce current[,] was a major breakthrough in the history of electronics. (Yes, it’s a long way from the subject “discovery” to the verb “was” but there should be no comma.)
Decreasing the radar operating frequency[,] increases the effective velocity coverage for the same sampling rate. (The whole phrase “Decreasing the radar operating frequency” is the subject of the verb “increases”—no comma.)
It can be assumed that[,] precipitation particles move with the air in their environment and are therefore good tracers for air motion. (Don’t know why people would put a comma here—does it feel like a pause?)
The separator between black mix and the zinc electrode[,] consists of a paper barrier coated with cereal or methyl cellulose. (No comma here either.)
That European refuse incineration costs are substantially lower than U.S. costs[,] is particularly evident when income from by-product recovery and salvage operations is included. (The whole clause, “That European refuse incineration costs are substantially lower than U.S. costs,” is the subject for the verb “is”—no comma.)
Use a comma between all independent clauses.
First, this: the term independent clause is just a fancy name for a complete sentence. They are exactly the same thing. The word “independent” is a tip-off. A clause is independent when it can stand on its own feet and convey a complete thought by using a subject and a verb. “I cried.” That’s a complete sentence. We can embellish it by adding various elements. “I cried when I realized I had to study grammar.” Okay, now let’s go back to calling complete sentences “independent clauses.”
Whenever you have a compound sentence (independent clauses joined by conjunctions such as and, but, yet, or, nor, for, whereas), put a comma before the conjunction. The length of the compound sentence does not matter. Below are some examples. (Conjunctions are italicized.)
Examples
The tank is made of aluminum, but the outer surface is protected by a spray-on foam.
By the mid-1970s, the free-spending ways of the Apollo Program were gone, and NASA now had to grapple with large technical challenges on a limited budget.
It first appeared that Hurricane Betsy would reach the eastern U.S., but a looping path took it around the tip of Florida and into the Gulf instead.
Gamma rays produce few pairs, but they travel farther.
One grate turns at 50 mph, but the others turn at 15 mph.
Type your name, and then press the Enter key. (This is an imperative (command) sentence—it qualifies as a compound sentence. There is an understood “you” subject for both parts of the sentence: You type your name, and then you press the Enter key.)
You should type your name and then press the Enter key. (In this case “you” is the subject for the compound verb—it’s the subject for both “should type” and “press.” This is not a compound sentence, and therefore there is no comma before “and.”)
Do not use a comma between two compound verb phrases.
Compound sentences can be tricky. There must be a complete sentence on both sides of the conjunction (that means a subject, a verb, and an object or complement—the works). Compare the following examples (subjects are italicized, and verbs are in bold; the bracketed commas indicate where erroneous commas are typically placed).
Examples
Offspring exposed to significant amounts of alcohol in utero are much more active than controls[,] and sometimes seem to fly around the room. (This is a compound verb phrase, not a compound sentence: “Offspring” is subject for both verbs.)
Plastic parts are not weldable[,] and must be repaired by other methods.
The observation and measurement of such small frequency shifts require excellent radar frequency-stability characteristics that are not usually found in conventional radar[,] but can be added without a drastic increase in equipment costs.
Pulse Doppler radar effectively samples the backscattered signal at the radar repetition rate[,] and therefore can provide unambiguous Doppler frequency observations only in the frequency range allowed by the sampling rate.
The manganese dioxide used in batteries is usually obtained from natural ore (mainly from Gabon, Greece, and Mexico)[,] but can be a synthetic product prepared by chemical precipitation or by electrolytic methods.
The last three sentences above probably seem incredibly long and in need of commas before and and but. Rather than break our rule (and remember it’s not breaking the rule that matters; it’s creating more and more exceptions that will drive us all crazy), why not break each lengthy sentence up to enhance clarity? See the revisions below.
Examples
The observation and measurement of such small frequency shifts require excellent radar frequency-stability characteristics that are not usually found in conventional radar. However, this same observation and measurement can be added without a drastic increase in equipment costs.
Pulse Doppler radar effectively samples the backscattered signal at the radar repetition rate. This type of radar therefore can provide unambiguous Doppler frequency observations only in the frequency range allowed by the sampling rate.
The manganese dioxide used in batteries is usually obtained from natural ore (mainly from Gabon, Greece, and Mexico). It can also be a synthetic product prepared by chemical precipitation or by electrolytic methods.
Use commas around all nonrestrictive elements.
Nonrestrictive elements are phrases and clauses that are nonessential to the meaning of the sentence. These elements can be taken out of the sentence without hurting its basic message. Use commas around these nonrestrictive elements. Examples are below.
Examples
Eighty percent of the work done by the heart is carried out by the left ventricle, which pumps blood into the arteries serving the organs and the tissues. (It’s nice of the writer to remind us what the left ventricle does, but the sentence could live without it; it would still make sense.)
The test produced a speed in the high-pressure hydrogen turbopump of 7000 rpm, which is 19 percent of design speed. (This is additional detail, not essential to the sense of the sentence.)
The Coriolis force, caused by the rotation of the earth, always acts at right angles to the pressure gradient in the northern hemisphere. (This is a helpful definition, but again is not essential to the sentence.)
The bulky equipment, although placed on a rolling cart, must always remain within 6 feet of the heart transplant patient. (Nonessential stuff—put commas around it.)
The formation of hurricane, a type of atmospheric vortex, involves the combined effect of pressure and circular wind. (The definition of a hurricane is not essential to the meaning of the sentence.)
Researchers also found that heavy drinkers—women drinking at least 1.6 ounces of absolute alcohol during pregnancy—have infants averaging 59 grams less than the infants of lighter drinkers. (Nonessential stuff—put commas around it, or in this case em dashes, which are commas by another name. Dashes serve to make the separated clause stand out more to the reader, and, in longer sentences, can enhance sentence clarity. Note that em dashes are different from hyphens. Do not use a hyphen in place of a dash. If you cannot create a dash using your keyboard, use two hyphens side by side to indicate a dash mark.)
Molecules may also have some degree of ordered as well as disordered motion, in which case the total energy is the sum of the mechanical and thermal energies. (Nonessential stuff—separate it with a comma.)
Do not use commas around restrictive elements.
Restrictive elements are phrases and clauses that a sentence desperately needs to make sense, to say what it means to say. If you take restrictive elements out of a sentence, you wreck the sentence. In the examples below, erroneous commas are bracketed to show that they do not belong in the sentence.
Examples
Problem: You can use the system[,] when the login prompt appears. (The way this sentence is punctuated implies that you can use the system any old time. The comma indicates that the clause beginning with “when” can be lifted from the sentence. But the sentence means to say that you can’t use the system until the login prompt appears.)
Revision: You can use the system when the login prompt appears. (The clause beginning with “when” is restrictive— it can’t be omitted from the sentence and therefore should not be set off by a comma. Now the sentence means that you can use the system only when the prompt appears.)
A turbopump is essentially a pump[,] that is turned by the action of a turbine that shares a common shaft with the pump. (It’s not any old pump; it’s one that does what the latter part of this sentence says it does. Imagine this sentence ending at “essentially a pump.”)
Eighty percent of the work[,] done by the heart[,] is carried out by the left ventricle. (Imagine this sentence without “done by the heart,” which is the restrictive element in this sentence. No commas here!)
A drop of water almost flattens out[,] when it is placed on a glass plate. (Imagine this sentence without “when it is placed on a glass plate,” which is the restrictive element here.)
In one study, 11 percent of the offspring[,] whose mothers consumed 2 to 4 drinks per day[,] showed partial features of fetal alcohol syndrome (FAS), while 19 percent of those whose mothers consumed 4 or more drinks per day showed FAS features. (Imagine this sentence without “whose mothers consumed 2 to 4 drinks per day” or without “whose mothers consumed 4 or more drinks per day.” The sentence simply wouldn’t make any sense. No commas!)
Use a comma before the “and” in a series of three or more words or phrases.
In series of three or more words or phrases, go ahead and put the comma before the and that occurs before the final element. You may have heard that this Oxford comma rule is optional. However, there are situations where the lack of the Oxford comma can cause confusion. And when you consider that using the Oxford comma cannot hurt the sense of the sentence, it makes sense to use it in all cases. Examples are below.
Examples
Instrument panels, bumper components, door liners, seat covers, and grille panels are the most common parts produced directly by automakers.
A 12-ounce can of beer, a 5-ounce glass of wine, and a mixed drink with 1.5 ounces of 80-proof liquor all contain approximately the same amount of alcohol.
The development years included designing the components for the Space Shuttle’s engines, testing the original designs, and retesting the redesigned components.
In humans, the period of rapid brain development begins at mid-pregnancy, peaks in the third trimester, and ends by the postnatal year.
Do not use a comma between words in a series of only two.
Be careful not to apply the Oxford comma rule to a series of only two elements. Watch out also for those situations where it looks like you have a series of three elements but it is actually a series of two noun phrases and a compound verb phrase (if this is meaningless—see the example).
Examples
We brought bread and cheese and read poetry.
(Sorry for the Dick-and-Jane sentence, but notice that “bread,” “cheese,” and “poetry” are not really a series. Because bread and cheese are nouns, and read poetry is an action (verb phrase), the meaning of the sentence is clear without the use of commas.
Punctuate series adjectives carefully.
It gets tricky knowing how to punctuate when two or more adjectives pile up in front of a noun. One fairly reliable technique is this: if you can switch the order of the adjectives or if you can insert and between them without making the phrase sound weird, then you can consider using commas. (Remember that in no case is there a comma between the final series adjective and the noun it modifies.)
Examples
He’s having his third mid-life crisis. Now he wants a new red sports car. (You couldn’t say “mid-life third crisis,” nor could you say “sports red new car”—so no commas are needed in or amongst these adjectives.)
Each door is held shut with an adjustable, spring-loaded door latch. (You probably could switch “adjustable” and “spring-loaded”—use a comma here.)
As each rack passes through the wash chamber, the dishes get a thorough soil-stripping wash and a final, automatic hot-water rinse. (You probably could switch “final” and “automatic”—use a comma here.)
The last two examples may have felt a bit “iffy” to you—the technique is only fairly reliable.
Note: This section doesn’t cover all comma rules; see a standard grammar handbook or online resource if you want to explore commas in greater detail. (Incidentally, you’ll notice a lot more flexibility in the rules in those standard reference books—they weren’t written for the technical-documentation context.)
