Commas keep English sentences readable, especially long, involved sentences. Without commas, readers wouldn’t know when to pause. But as the following rules show, correct placement of commas reflects the grammar and syntax of the language, not merely places to pause. See PUNCTUATION for information on mandatory and optional uses of commas.
Commas separate complete thoughts joined by these simple conjunctions: and, but, or, for, nor, so, yet:
- He was a Russian linguist in communications intelligence, and he has logged over 5,000 hours as a C-130 navigator in the Air Force.
- Visophane has been marketed abroad since 2005, but it was not approved for the local market until May of last year because of insufficient clinical trials.
NOTE 1: You may omit this comma if both complete thoughts are short:
- The chairman resigned and the company failed.
The simple conjunctions cited above are called coordinating conjunctions. When they link two complete thoughts, the resulting sentence is called a compound sentence. See CONJUNCTIONS, SENTENCES, and BRITISH ENGLISH.
NOTE 2: If you use any other transitional or connecting word (however, furthermore, consequently, and so on) to join two complete thoughts, use a semicolon. See SEMICOLONS and TRANSITIONS.
Commas separate items in a series consisting of three or more words, phrases, or even whole clauses:
- Control Data’s Integrated Support Software System provides compatibility between tools and workers, consistent tool interfaces, ease of learning, user friendliness, and expandability.
- The user may also return to the control program to perform such other functions as database editing, special report generation, and statistical analyses.
- The Carthage-Hines agreement contained provisions for testing the database, cataloguing the findings, creating a more user-friendly software package, and marketing any new software developed jointly.
NOTE 1: A comma separates the last two items in a series, even though these items are linked by a conjunction (and in the above examples, but the rule applies for any conjunction). This comma was once considered optional, but the trend is to make it mandatory, especially in technical and business English. Leaving it out can cause confusion and misinterpretation. See PUNCTUATION and BRITISH ENGLISH.
NOTE 2: If all of the items in the series are linked by a simple conjunction, do not use commas:
- The user may also return to the control program to perform such other functions as database editing and special report generation and statistical analyses.
NOTE 3: In sentences containing a series of phrases or clauses that already have commas, use semicolons to separate each phrase or clause:
- Our legal staff prepared analyses of the Drury-Engels agreement, which we hoped to discontinue; the Hopkinson contract; and the joint leasing proposal from Shell, Mobil, and Amoco.
See CONJUNCTIONS and SEMICOLONS.
Commas separate long introductory phrases and clauses from the main body of a sentence:
- Although we are new to particle scan technology, our work with split-beam lasers gives us a solid experiential base from which to undertake this study.
- For the purposes of this investigation, the weapon will be synthesized by a computer program called RATS (Rapid Approach to Transfer Systems).
- Oil production was down during the first quarter, but when we analyzed the figures, we discovered that the production decline was due to only two of our eight wells.
NOTE 1: In the last example, the when we analyzed clause does not open the sentence, but it must still be separated from the main clause following it. It introduces the main thought of the last half of the sentence.
NOTE 2: If the introductory thought is short and no confusion will result, you can omit this comma:
- In either case the Carmichael procedure will be used to estimate the current requirements of the preliminary designs.
Commas enclose parenthetical expressions.
Parenthetical expressions are words or groups of words that are inserted into a sentence and are not part of the main thought of the sentence. These expressions describe, explain, or comment on something in the sentence, typically the word or phrase preceding the parenthetical expression:
- The transport will, according to our calculations, require only 10,000 feet of runway.
- The survey results, though not what we had predicted, confirm that the rate of manufacturer acceptance will exceed 60 percent.
Parentheses and dashes may also enclose parenthetical expressions. Use commas most of the time, but when you want to make the expression stand out, enclose it with parentheses (which are more emphatic than commas) or dashes—which are more emphatic than parentheses. See PARENTHESES and DASHES.
Commas separate nonessential modifying and descriptive phrases and clauses from a sentence, especially those clauses beginning with who, which, or that:
- These biocybernetic approaches, which merit further investigation, will improve performance of the man/machine interface.
In this sentence, which merit further investigation is not essential because the reader will already know which biocybernetic approaches the sentence refers to. The clause beginning with which is nonessential and could be left out:
- These biocybernetic approaches will improve performance of the man/machine interface.
If several biocybernetic approaches were listed, however, and if the writer needed to identify only those meriting further investigation, the clause would be essential, could not be left out, and would not take commas:
- Improving the performance of the man/machine interface meant identifying those biocybernetic approaches that merit further investigation.
The that in the preceding example commonly introduces essential clauses, although which sometimes appears. See that/which in WORD PROBLEMS.
Modifying or descriptive clauses should always follow the words they modify. If they cannot be removed from the sentence without changing the meaning, they are essential and must not be separated by commas from the word they modify. If they can be removed, they are nonessential and must be separated by commas from the main thought in the sentence:
- Essential: She is the Dr. Gruber who developed analytical engine compressor stability models for NASA.
She is the Dr. Gruber does not make sense as an independent statement. The descriptive clause beginning with who is essential and therefore cannot be separated by a comma from Gruber.
- Nonessential: Our Design Team Leader will be Dr. Janet Gruber, who developed analytical engine compressor stability models for NASA.
Our Design Team Leader will be Dr. Janet Gruber does stand alone as a complete and independent thought. In this case, the descriptive clause beginning with who is nonessential. Separating it from Gruber with a comma shows that it is additional and nonessential information. Note that a comma would follow NASA if the sentence continued. See PRONOUNS for a discussion of relative pronouns.
Commas separate two or more adjectives that equally modify the same noun:
- This design features an advanced, multidose oral therapy.
NOTE: If two or more adjectives precede a noun, however, and one adjective modifies another adjective—and together they modify the noun—you must use a hyphen:
- They had designed a no-flow heat exchange.
A good test for determining whether two or more adjectives equally modify a noun is to insert and between them. If the resulting phrase makes sense, then the adjectives are equal, and you should use commas to replace the ands:
- old and rusty pipe (therefore, old, rusty pipe)
- old and rusty and steam pipe (The and between rusty and steam makes no sense. Therefore, the phrase should be old, rusty steam pipe.)
See HYPHENS and ADJECTIVES.
Commas separate items in dates and addresses:
- The proposal was signed on March 15, 2007.
- Contact Benson Pharmaceuticals, Lindsay, Indiana, for further information.
NOTE: A comma follows the day and the year when the month and day precede the year. However, when the date consists only of month and year, a comma is not necessary:
- The final report will be due January 14, 2011, just a month before the board meeting.
- The final report will be due in January 2011.
When the date appears in the day-month-year sequence, no commas are necessary:
- The report is due 14 January 2011.
Commas separate titles and degrees from names:
- The chief liaison will be Roger Hillyard, Project Review Board Chairman.
- Mary Sarkalion, PhD, will coordinate clinical studies.
- Clinical studies will be the responsibility of Mary Sarkalion, PhD.
NOTE: When the degree or title appears in the middle of a sentence, commas must appear before and after it.
Commas follow the salutation in informal letters and the complimentary closing in all letters:
- Dear Joan,
See COLONS and LETTERS.
Commas enclose in text the names of people addressed:
So, Bob, if you’ll check your records, we’ll be able to adjust the purchase order to your satisfaction.
Commas (or a comma and a semicolon) set off (enclose) the following transitional words and expressions when they introduce sentences or when they link two complete thoughts: accordingly, consequently, for example, for instance, further, furthermore, however, indeed, nevertheless, nonetheless, on the contrary, on the other hand, then, thus:
- Consequently, the primary difference between CDSP and other synthesis programs is development philosophy.
- Synthesis programs are now common in industry; however, CDSP has several features that make it especially suitable for this type of study.
- Synthesis programs are now common in industry; CDSP has, however, several features that make it especially suitable for this type of study.
NOTE: A few of these transitional words (however, thus, then, indeed) are occasionally part of the main thought of the sentence and do not form an actual transition. When such is the case, omit the punctuation before and after the words:
- However unreliable cross-section analysis may be, it is still the most efficient means of scaling mathematical models.
- Thus translated, the decoded message can be used to diagram nonlinear relationships.
Commas, like periods, always go inside closing quotation marks. Commas go outside parentheses or brackets:
- The specifications contained many instances of the phrase “or equal,” which is an attempt to avoid actually specifying significant features of a required product.
- Thanks to this new NSAID (non-steroidal anti-inflammatory drug), posttraumatic or postoperative conditions were significantly reduced.
NOTE: British usage places commas and periods inside or outside the quotation marks, depending on whether they are or are not part of the quotation. See SPACING, BRITISH ENGLISH, and QUOTATION MARKS.