Commas are confusing because they are used in many ways. However, the basic principle to using commas is simple: Use commas to separate clauses and phrases within sentences that have their own meaning.
The “rules” for commas below are broadly, but not universally, accepted. However, a careful writer considers two central issues:
- Reader understanding and
The comma guidelines below will help readers understand your message in many cases. However, even if they are not necessary to improve reader understanding, follow them for consistency. Consistency is a characteristic of professional technical writing.
The commas help the reader find each unique item (or group of items) in a series by separating them.
Example: School officials are dismayed by poor grades, low attendance, and high drug use.
2. Joining Sentences
You can join two complete sentences with coordinating conjunctions. (The entire set of coordinating conjunctions is for, and, nor, but, or, yet, and so. Together, these create the acronym FANBOYS.) The comma lets the reader know when one point is complete and the next will begin. This comma use only applies when you have complete sentences on either side of the conjunction.
Example: The screen inverter stopped working, and the motherboard began to smoke.
3. Introductory Descriptions
An introductory description is before the subject and describes the main verb in some way, such as when, where, how, and why. The comma at the end of the description signals the reader that the main point of the sentence is about to begin. For consistency, do this with even short introductory descriptions. In the following example, the introductory description is underlined.
Example: Following the symposium, participants collaborated on projects.
Technical and academic writing are no places for interjections. However, if you use an interjection, separate it from the rest of the sentence with commas. In the following example, the interjection is underlined.
Example: Hurrah, the project is finally complete.
An appositive renames or restates the person or thing you just wrote. It is equal and indicates the identical information. Appositives are separated from the rest of the sentence with commas to indicate that the information is a restatement. In the following example, the appositive is underlined.
Example: A Do Not Resuscitate order, a form of advanced directive, is often established during end-of-life care.
6. Asides, Interjected Comments, Parenthetical Expressions
While writing a sentence, you may want to include information that is not directly related to the main point of the sentence. To indicate that you are going “off topic,” and to indicate when you are returning to the main topic, separate the information with commas. If you can put these expressions in parentheses, you can use commas, instead. The information is not essential for understanding the point you are trying to make and, therefore, can be safely separated from the rest of the sentence.
Example: The musical trends of the 20th Century, as determined by a survey of published sheet music, indicates a correlation between tempo and public confidence in the national economy.
7. Providing Examples
When you are providing examples of some fact or concept, you create a form of parenthetical expression. Thus, examples, too, are separated from the rest of the sentence with commas.
Example: Alternative forms of transportation, such as bicycles and roller skates, are common but not popular in suburban areas.
8. Adjective Pairs
When you have two or more adjectives or other descriptions immediately before the word they describe, you may need to separate them with commas. If you can change their order, and if you can put and between them without changing the meaning of the sentence, the adjectives are called coordinate adjectives and need to be separated by a comma. The full explanation is far more technical, but if your adjectives meet these two conditions, you will place the commas correctly. The comma indicates that one adjective does not describe the following adjective in some way but that the adjectives equally describe the thing that follows.
Example: An empowered, supported student will find education exciting.
9. Non-restrictive Phrases and Clauses
A non-restrictive phrase or clause does not tell the reader which thing or person you are describing. Rather, a non-restrictive phrase or clause provides an “off-topic” description that is not necessary to understand the main point of the sentence. Separate all non-restrictive phrases and clauses from the rest of the sentence with commas. If the information is necessary for the reader to know which thing or person you are describing, do not use commas.
Example: The maple tree, which harvesters tap for their sap to make syrup, produces seeds in pairs.
Example: The CEO of Widgets.com, who began his career as a shop clerk, has a net worth of $1 million.
If you include the day, month, and year in the date, put commas around the year. If you do not include the day, you do not need the commas.
Example: On July 1, 2004, the publisher will release the new book series.
11. Final descriptive phrases and clauses
A sentence may end with a final descriptive phrase or clause. If the descriptive phrase or clause relates only to the words immediately prior to the description, you do not need a comma. However, if the description relates to the entire sentence, use a comma to separate it. This shows that the description relates to the entire sentence, not just one part of the sentence. (In the following example, the underlined description relates to the entire information in the main sentence. Without a comma, the sentence would indicate that based on their website notice describes why they postponed the ceremony.)
Example: The city council postponed the ceremony, based on their website notice.
12. Quoted Text
In U.S. English, the final comma or period goes inside the ending quotation mark—even if it looks awkward or is not part of the quoted material.
Example: According to business researchers, election results have a “noticeable effect on stock prices,” and the entire stock market generally “finds a new balance.”
Incorrect Comma Placement
Do not use commas in the following places within sentences.
1. Before or (in either…or expressions), nor (in neither…nor expressions), but also (in not only…but also expressions) expressions, and before the second part of other, similar two-part expressions. Example: “The launch will occur either on Friday [no comma] or on Saturday.”
2. Between the subject and predicate, unless the subject ends with an expression (such as an appositive) that requires commas. Example: “A substitute for natural wood products shipped from Finland [no comma] expands the market base.”
3. Between two parts of a compound predicate. Example: “Sound waves can produce heart palpitations in people within the wave cone [no comma] and create pressure on sinus tissues.
4. Between two objects of a verb. Example: “We use ocean currents to create electricity [no comma] and air currents to generate electromagnetic fields.
This concise comma guide was adapted from the forthcoming Bowman’s Concise Guide to Technical Writing.