Improper comma use is one of the most common grammatical errors in scientific writing, likely due to the prevailing thought that commas must be placed where the speaker would naturally pause in a sentence. However, did you know there are objective RULES for where to place commas that don’t rely on an individual’s personal preference for how a sentence reads? If you have ever struggled with properly populating your written document with a forest of commas, this post is for you. I’ll explain some general guidelines for when commas are necessary, when they are dispensable, and when they are dispensable but probably should be necessary (I’m looking at you Oxford comma).
Rule 1: Put a comma between two independent clauses separated by “FANBOYS” connectors: For, And, Nor, But, Or, Yet, So.
The key word here is INDEPENDENT. If two clauses, each with its own subject and verb, could stand alone as separate sentences, then a comma is needed when combining the two clauses with a FANBOYS connector: for, and, nor, or, yet, so. Place the comma before the connector word.
INCORRECT: The microscope is broken and I know who did it.
CORRECT: The microscope is broken, and I know who did it.
An independent clause followed by a dependent clause (one that cannot stand on its own) does not need a comma when separated by a FANBOYS connector.
INCORRECT: The microscope is broken, but will be fixed soon.
CORRECT: The microscope is broken but will be fixed soon.
Beware of fake FANBOYS: However, Moreover, Therefore, and other conjunctive adverbs. These connectors need a semicolon in front of them and a comma after them.
INCORRECT: The microscope is broken, however I know who did it.
CORRECT: The microscope is broken; however, I know who did it.
Rule 2: When starting a sentence with an introductory word or dependent clause, use a comma to separate it from the rest of the sentence.
When a dependent clause precedes an independent clause, a comma is necessary (this sentence is an example).
INCORRECT: Despite its efficacy the drug is still unavailable to most people.
CORRECT: Despite its efficacy, the drug is still unavailable to most people.
A comma is also needed when a single introductory word is used to start a sentence.
INCORRECT: Thus the experimental results do not support our hypothesis.
CORRECT: Thus, the experimental results do not support our hypothesis.
Rule 3: Use a comma to set off non-essential words or phrases.
This comma rule can be somewhat subjective depending on what the writer considers is necessary for proper understanding of the sentence. You may need to ask yourself, “Can this word or phrase be deleted, and the meaning of the sentence stays the same?” If the answer is no, then the word or phrase needs to be set apart by commas.
INCORRECT: The cell which was infected would likely die soon.
CORRECT: The cell, which was infected, would likely die soon.
Clauses that follow the word “that” are always considered essential and should not be preceded by commas.
INCORRECT: The cell, that was infected, would likely die soon.
CORRECT: The cell that was infected would likely die soon.
In the examples above, the meaning of the sentence changes subtly depending on the usage of commas. In the first example, the cell is going to die. The fact that it is infected is extra information and not necessary for the overall meaning of the sentence. In the second example, it is important that the reader understands which cell (the infected one) is about to die.
In scientific writing, this rule often applies to nouns or phrases that provide an alternative name or identity for a gene, protein, etc. If the alternative name is not necessary for the understanding of the sentence, then it needs to be set apart by commas.
INCORRECT: The gene responsible for the phenotype LKB1 is conserved among animals.
CORRECT: The gene responsible for the phenotype, LKB1, is conserved among animals.
INCORRECT: The gene, LKB1, is conserved among animals.
CORRECT: The gene LKB1 is conserved among animals.
Rule 4: Use a comma to separate two adjectives when the order of the adjectives is interchangeable.
To decide if a comma is needed, mentally put “and” between the two adjectives or reverse their order. If the sentence still makes sense, add a comma.
INCORRECT: The fluffy energetic mouse was running on its wheel.
CORRECT: The fluffy, energetic mouse was running on its wheel.
INCORRECT: I left my favorite, wool sweater in lab.
CORRECT: I left my favorite wool sweater in lab.
Rule 5: Use commas to separate words and word groups in a series of three or more items.
INCORRECT: The experiment requires forceps dissecting pins and stable hands.
CORRECT: The experiment requires forceps, dissecting pins, and stable hands.
ALSO CORRECT: The experiment requires forceps, dissecting pins and stable hands.
The last comma in a series is referred to as the Oxford comma (or serial comma) and is considered optional. However, leaving it out can drastically change the meaning of the sentence.
TECHNICALLY CORRECT BUT CONFUSING: This thesis is dedicated to my parents, Gregor Mendel and God.
ALSO CORRECT AND LESS CONFUSING: This thesis is dedicated to my parents, Gregor Mendel, and God.
The Oxford comma is required by some style guides, such as the MLA Style Manual and the American Medical Association Manual of Style, while other styles actively discourage its use, including the AP Stylebook. For scientific writing, follow the journal’s guideline for Oxford commas if stated. Otherwise, use of the comma is left up to your personal preference (just remain consistent throughout the article).
Rule 6: Use a comma after “i.e.” and “e.g.”.
These abbreviations appear often in scientific writing and usually are followed by a dependent clause or a list; therefore, they need to be followed by a comma.
Mitochondria function as energy producers (i.e., powerhouses of the cell).
Organelles (e.g., mitochondria, the ER, and Golgi) are important for the cellular functions.
There are additional rules for when to use commas in more specific contexts, such as in dates or addresses, but the above rules apply to the majority of scientific or technical writing. See the additional resources below if you want to delve further into the fascinating world of proper comma usage!
References and other resources: