More about commas? Given the wealth of information about commas presented in the last Word of the Week article , this is a fair question.
NASA Langley's Grammar, Punctuation, and Capitalization: A Handbook for Technical Writers and Editors ,1 referenced in our last article, noted that "the primary functions of the comma are to separate and to enclose the elements of a sentence." In our last article we dealt with commas that separate, now we will focus on commas that enclose.
It is standard form to omit commas whenever possible--so long as there is no danger of ambiguity when you do so. However, it's much easier to comfortably omit a comma when you are sure of the rules for the use of commas. Follow the links in the headings below for more detailed explanations 1 of commas that enclose the elements of a sentence.
The most important thing to remember about commas that enclose (other than when to use them) is that an enclosing comma must always have a partner. The partner does not need to be another comma; it can be a colon, semicolon, period, question mark, or exclamation mark.
"A nonrestrictive modifier does not affect the meaning of the basic sentence; it could be removed from the sentence without altering meaning."1
Here a comma and period enclose:
Materials for the spacecraft were mainly selected on the basis of availability, cost, weight, and reliability, which relegated color to a back-burner position.
In the following sentence, two commas enclose the nonrestrictive modifier:
NASA's largest launch vehicle, which is as tall as a 20-story building, weighs about 2 million pounds.
An appositive is another way of referring to something or someone. In the following sentence, "our new postal carrier" is an appositive.
"Sam, our new postal carrier, is always on schedule."
Commas are used to set off words used as appositives or contrasts:
Hydrogen, the cryogenic fluid of choice, is the most difficult cryogenic fluid to handle.
The next sentence gives an example of a contrasting phrase:
This approach provides simple, yet highly reliable, temperature control for all situations.
Nonrestrictive phrases, such as parenthetical phrases, rhetorical adverbs, introductory words, and antithetical phrases, are usually set off by commas.
The design constraints imposed by the Martian environment, such as those listed previously, can greatly affect the shape of the aircraft.
The elapsed time between tests was uniform; however, the test times varied widely.
A phrase with termination that reads back to a previous phrase is enclosed with commas:
The static pressure measurement at three of these stations, if not all, is anticipated to be the same.
The experiment panels should be as cold as, if not colder than, the control panels.
A phrase that has a noun modified by a participle and which is grammatically unconnected with the rest of the sentence is a nominative (subjective) absolute. Such phrases, which are nonrestrictive, usually tell when, why, or how something happened.
All four bottles were decoupled from the mainframe, the radiative decoupling of the main tank already having been performed.
The fuel having already been selected, we concentrated on finding a suitable engine.
There are many conventional uses of the comma, such as in dates (February 26, 1998), geographical names (Cleveland, Ohio), names (Dr. Martin Luther King, Jr.), and addresses. One such use, often debated in technical work, is the comma that is used to separate thousands in numbers of five or more digits (34,566). Our style at NASA Lewis is to separate these numbers by a space (34 566) because in some countries the comma indicates a decimal point. Follow the link in this section for a good refresher on such uses of the comma.