Punctuation: Commas

Punctuation is a good example of this effort to use clearly defined rules in technical writing. In journalistic punctuation style, you punctuate according to what you feel are the needs for clarity. But this is likely to be viewed differently by different people. Therefore, punctuation style in technical writing is based on the structure of the sentence.

Use a comma after all introductory elements. Any element, regardless of the 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.) Here are some 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 originate there with great frequency.
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.

(Punctuate even short introductory phrases like this and the next two sentences.)
Using this concept, Faraday arrived at a relation between the changing flux and the induced electromagnetic field.
Today, the computer consortium of IBM, Mototrola, and Apple is announcing its new PowerPC chip.

Doublecheck commas between the parts of a sentence. A single comma should never break the flow of the main subject, verb, and object or complement of a sentence. Instead, commas should occur in pairs. Here are some examples (the bracketed commas indicate where commas are typically but mistakenly placed):

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. Whenever you have a compound sentence (those are the ones joined by and, but, yet, or, nor, for, whereas), put a comma before the conjunction (the words I just listed in italics). Length of the compound sentence does not matter. Here are some examples (conjunctions are italicized):

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 her around the tip of Florida and into the Gulf instead.

Gamma rays produce few pairs, but they travel farther.

(These next three examples are short, but to keep life as simple as possible we punctuate them.)

One grate turns at 50 mph, but the others turn at 15 mph.

Type your name, and then press the Enter key.

(These are two imperative sentences—this qualifies as a compound sentence. But check out the next example.)

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 phrase. Watch out about what you think are compound sentences. A complete sentence has to be on both sides of the conjunction (that means subject, verb, object, or complement—the works). Compare the following examples (subjects are italicized, and verbs use a different color; the bracketed commas indicate where commas are typically but mistakenly placed)):

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 to you and needy of commas at 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 split these into two sentences each as in the following?

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 grammar of the sentence. These elements can be taken out of the sentence without hurting its basic message. Use commas around these nonrestrictive elements. Here are some 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.
(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 rom, 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.

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 dashes, which are commas by another name.)

Adding waterproofing material to a fabric increases the contact angle, making the fabric water-repellent.
(Nonessential stuff—put commas around it!)

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—put commas around it!)

Do not use comma 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!

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.)
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 punctuated. Now the sentence means that you can use the system only when the prompt appears.)

Here are some additional examples of this rather tricky rule:

A turbopump is 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. No commas need apply!)

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. 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 series-and comma rule is optional. However, there are situations where the lack of the series-and comma can cause confusion. And when you consider that using the series-and comma cannot hurt the sense of the sentence, it makes sense to use it in all cases. Here are some 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 involved 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 a series of only two. Be careful not to apply the series-and 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).

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 in a series. No commas for either "and" here.)

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.)

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 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.)

These last two examples may have felt a bit "iffy" to you—the technique is only "fairly" reliable.

Note: This doesn't cover all commas rules; see a standard handbook like the ones mentioned in the introduction to this chapter.. (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.)

Punctuation: Colons

Although the colon has other uses in writing, its most important function is to act as a signal to the reader—it says something like "Okay, reader! Here it comes!" In the first example, notice the words before the colon make a complete statement—at least grammatically:

To make a kite, you need the following items: string, paper, thin sticks, glue, and scissors.

The main engines of the Space Shuttle consist of six main components: the external tank, the low-pressure turbopump, the high-pressure turbopumps, the preburners, the combustion chamber, and the nozzle.

Hurricane size is expressed in three ways: the strength of the maximum winds, the diameter of the hurricane-force winds, the diameter of the gale-force winds, and the overall size the cyclone circulation.

To make a metal dashboard, three steps are required: (1) the metal must be stamped; (2) the texture must be stamped into the metal; and (3) the part must be painted.

Notice in the last example that the first sentence introduces a series of complete sentences. You can use the colon to connect two complete sentences—as long as the first sentence introduces or prepares for the second. Here are some examples of this possibility:

The grades of the students in the caffeine research project told a dramatic story: the higher the caffeine intake, the lower the grades, both for semester and overall grade point average.

In general, shelf-life increases as the cell size of the battery becomes smaller: with well-constructed cells, shelf-lives of three years with a No. 6 telephone cell and ten years with a penlight cell are possible.

The line-of-sight in a communication satellite can be a problem: communication satellites can see the earth's surface only between about 83 degrees north latitude and 83 degrees south latitude.

Many of the new applications of microcomputers are "interactive": there is frequent interaction between the computer and one or more users.

However, don't use a colon inside a complete sentence. It should connect only complete sentences to complete sentences or connect complete sentences to lists.

Problem: The typical Doppler velocity sensor consists of[:] a transistor, an antenna, and a receiver.
Revision: The typical Doppler velocity sensor consists of a transistor, an antenna, and a receiver.

Problem: Three significant types of generating plants are[:] hydroelectric, fossil-fuel-electric, and nuclear-electric.
Revision: Three significant types of generating plants are hydroelectric, fossil-fuel-electric, and nuclear-electric.

Problem: You will need[:] string, paper, thin sticks, glue, and scissors, to make a kite.
Revision: You will need the following items—string, paper, thin sticks, glue, and scissors—to make a kite.

Look at this last example closely: the grammatical core of the sentence is "You will need the following items . . . to make a kite." You don't want to break up the core grammar of a sentence this way with a colon.


The semicolon could be called a strong comma. Its two main uses are to connect two (or more) sentences that seem very closely related and to clarify the punctuation of a series of items that have their own internal commas.

You may have had some unhappy encounters with run-ons and comma splices in the past. These two "comma faults" usually result from the writer's sense that the sentences involved in the problem are very closely related—the full stop signaled by the period seems like too full of a stop. (It's almost like music; makes you wonder why we don't have the equivalent of whole, half, quarter, and eighth rests in punctuation.) Often, these run-on sentences and comma splices can be fixed by substituting a semicolon for the offending comma.

But not always. Some writers go way overboard in sensing close relations between sentences. Well, yes, every sentence in a document is related to every other—they ought to be! But they need to be reeeaaally closely related. Here are some examples:

"Plaque-fissuring" refers to the formation of an opening from the lumen to the intima; it leads to an intra-intimal thrombus containing not just red cells but mainly fibrin and platelets.

In 1940, philanthropy accounted for 24 per cent of the total operating budget of nonprofit hospitals in New York City; in 1948; it had dropped to 17 per cent.

Gray mold is one of the most important fungal diseases in Italian viticulture; its growth causes serious production losses and adversely affects wine quality.

The other use of the semicolon worth noting here is how it can clarify items in a series that have commas within them already:

Injury caused by pollutants can easily be mistaken for injury caused by other stresses; or, just the opposite, injury symptoms from adverse temperature or moisture relations may resemble, and can be incorrectly attributed to, air pollutants.

Possible research areas announced recently have included genetics, fermentation microbiology, and immobilized biocatalysts; but environmental biotechnology, such as metal recovery and waste recycling, is also included.

A typical membrane potential of about one-tenth of a volt sounds relatively small; but, because it occurs across a membrane that is only about 10 nanometers thick, it represents an enormous voltage gradient of about 10 million volts per meter.

The heart undergoes two cardiac cycle periods: diastole, when blood enters the ventricles; and systole, when the ventricles contract and blood is pumped out.

An organization may be functional, with responsibility assigned on the basis of buying, selling, promotion, distribution, and other tasks; production-oriented, with production managers for each product category and brand managers for each individual brand in addition to functional categories; or market-oriented, with managers assigned on the basis of geographical markets and customer types in addition to functional categories.

Electric power substations are used for some or all of the following purposes: connection of generators, transmission or distribution lines, and loads to each other; transformation of power from one voltage level to another; interconnection of alternate sources of power; and detection of faults, monitoring and recording of information, power measurement, and remote communication.

A common misuse of the semicolon is to plunk it down between what appear to be two complete sentences:

Problem: The slide rule was an important device for scientists and engineers for many years[;] although its use has all but vanished since the advent of the pocket calculator.
Revision: The slide rule was an important device for scientists and engineers for many years, although its use has all but vanished since the advent of the pocket calculator.

(The "although" clause is not complete; it can't stand on its own.)


Pity the poor apostrophe—it's practically an endangered species. The problem with the apostrophe is that it has some conflicting tasks: it is used primarily to show possession, mark contractions, and, minimally, to show plurals. But people have gotten it all mixed up. For example, the likes of "John love's Mary" was becoming pretty common in telephone booths before the rise of the cell phone. A scant two to three hundred years ago, people didn't even use apostrophes (yes—a world without apostrophes!). But the thing does add precision to writing; it does prevent confusion. The rules are stupidly simple; here they are:

Now, there are others rules involving apostrophes such as for contractions or for quotes within quotes, but we'll leave those for the reference books to handle.


Someone once said, "Take hyphens seriously and you will surely go mad." They weren't lying! (By the way, this previous sentence has a pronoun-reference problem.)

Hyphens are supposed to keep us from misreading things and show us how words in complex phrases relate to each other. The problem is that the rules for hyphens just cannot be applied absolutely consistently—you end up hyphenating everything including the kitchen sink. Professional editors end up keeping long lists of exactly which word pairs they will hyphenate in a specific document (so that they don't end up in therapy).

Hyphens do matter, however (save the hyphen!). Our language culture seems to be very "into" piling up ambitious noun phrases. These sentences verge on having a problem called "noun stacks." To read this kind of stuff, we need hyphens—they show us what goes with what. Hyphens show that a pair of words is acting as a unit and must be read that way. The common types of unit modifiers—which are two or more words acting as a unit—are discussed in the following (but it's by no means exhaustive):