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2005, Q3 (September 19, 2008)
By Andrea Wenger, Senior Member, Carolina Chapter

Thomas Mann described the writer as somebody for whom writing is more difficult than it is for other people. Nowhere is this truer than for comma use: while most folks float along blithely putting commas in or leaving them out at whim, we agonize over every squiggle. Why? Because we understand that the presence or absence of a comma can change the meaning of a sentence. In our line of work, unclear sentences can have dire consequences for our readers. So we worry.

In grade school, I was taught never to use a comma before a conjunction. Somehow, I managed to earn a degree in English without anyone dispossessing me of that silly idea. So I avoided using commas before conjunctions even when instinct told me I needed one. But when I became a technical writer, I discovered the rules on comma use put forth in The Chicago Manual of Style and was freed of my misapprehension.

Most importantly, clarity trumps all. If a sentence is ambiguous without a comma, then put one in. If a comma hinders the logical flow of ideas, then leave it out. But to develop the skills to make subjective calls, writers must first become fluent in the proper use of the comma. A comma, like its cousin the semicolon, compartmentalizes ideas. It gives the reader pause so that thoughts don't run into one another.

Commas with Adjectives

When two or more adjectives modify a noun, place a comma between the adjectives only if the comma could be replaced by the word "and":
  • the red, flashing LED (the LED is red and flashing)
  • the red push button (the push button is red)

Commas with Subordinate Phrases and Clauses

When a subordinate phrase or clause comes at the beginning of the main clause, separate it with a comma. When the same phrase or clause appears after the main clause, don't use a comma:
  • After work on Friday, I went shopping for shoes.
  • I went shopping for shoes after work on Friday.

With a short adverbial phrase at the beginning of a sentence, you can omit the comma:
  • On Friday I shopped for shoes.

Use commas around a non-restrictive phrase, but not a restrictive one, depending on the intended meaning of the sentence:
  • The new mall, across the street from my office, offers many fine shoe stores.
    (There's just one new mall, and it's across the street from my office—lucky me!)
  • The new mall across the street from my office offers many fine shoe stores.
    (There's also a new mall downtown, but its selection of shoes is only fair.)

A pair of commas enclosing the subordinate phrase or clause lifts and separates, providing definition and support. Be sure to place a comma at both ends of the phrase though.

(You wouldn't want to be lopsided, would you?) Consider the following example
New product features provide increased functionality, ensuring that networks run efficiently and remain immune from external dangers and improved security.

A comma after the word "dangers" would make it clear that the product doesn't protect against improved security. (An even better choice would be enclosing the phrase in em dashes instead of commas.)

Serial Constructions

When is it proper to use a comma before a conjunction? Not all style guides agree. Chicago calls for the serial comma (that is, a comma before the conjunction in a series or list), while the Associated Press Stylebook does not. Of course, follow the style guide used by your company or client. But if no style guide has been established (horrors!), I recommend using the serial comma. It doesn't do any harm, and sometimes a serial comma is essential to the meaning of the sentence
Key applications are conveyors, packaging and material handling equipment and semiconductor fabrication tools.

A comma after "equipment" would make it clear that "packaging" doesn't standalone; it modifies "equipment."

When three or more items in a series are all joined by conjunctions, don't use commas:
  • My new black shoes also come in tan or navy or white.

If any of the items in a series contain internal punctuation, separate them with a semicolon instead of a comma:
  • My new black shoes also come in tan; in navy with a shiny golden buckle; or in white, with or without a buckle.

Commas with Compounds

Generally, use a comma before the conjunction in a compound sentence but not in a compound predicate:
  • I love my new shoes, but my husband thinks I paid too much.
  • I love my new shoes and don't care how much they cost.

If at least one independent clause in a compound sentence contains internal punctuation, you can use a semicolon instead between the independent clauses
I love my new shoes; but my husband, the big cheapskate, thinks I paid too much.
(Notice also the use of commas to enclose an appositive—"the big cheapskate" being another way of saying "my husband.")

When the independent clauses in a compound sentence are short, you can omit the comma. Conversely, when a compound predicate is long or complex, you can include the comma:
  • I shopped and my husband cooked.
  • My husband roasted Cornish game hens stuffed with rosemary dressing, and had dinner waiting when I came home from the mall.

Don't use a comma between the independent clauses of a compound sentence when no conjunction is present; use a semicolon instead.

Considerations for Technical Communicators

In technical writing, it can be difficult to distinguish a compound sentence from a compound predicate. We often write in the second person and in the imperative mood; so the subject of the sentence, "you," is implied. Use of commas depends on whether there is an implied "you" after the conjunction:
  • Loosen the captive screws and remove the cover.
  • Loosen the captive screws in the four corners, and remove the cover.

The semicolon is sparsely used these days; perhaps folks think it's stuffy and old-fashioned; but it's a useful fellow; it has the power to keep a sentence endlessly aloft. As technical writers, we must cater to the tendency of Americans to read the step, then do the step.

A period signals users that they have all the information required to do the step; a semicolon warns them to continue reading.

The comma's nature is unassuming, its purpose to introduce quietness into a sentence. Yet it's difficult to imagine communication without it. We'd be doomed to using simple sentences or to tossing all our ideas onto a jumbled heap for our readers to sort out if we dismissed commas completely. Proper comma use frees readers' minds so that they can complete their tasks efficiently and get on with their lives. That is the greatest gift we can give them.

Andrea Wenger can be reached at andrea dot wenger at us dot schneider-electric dot com. End of article.

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