Cohesion and Clarity
12- 39
Avoiding Common Comma Errors
When to Use Commas
Use commas between independent clauses that are linked with a coordinating conjunction — and, but, or, nor, so, for, yet (an independent clause has a subject and a verb and could stand alone as a sentence): Not: Most patients tolerated the drug regimen well but three patients required hospitalization for grade 4 toxic effects. Correct: Most patients tolerated the drug regimen well, but three patients required hospitalization for grade 4 toxic effects.
Use commas between items in a series if there are 3 or more items:
Correct: The patient received fluorouracil, doxorubicin, and cyclophosphamide. But: The patient received doxorubicin and cyclophosphamide.
Use commas to set off nonrestrictive clauses or phrases (nonrestrictive clauses or phrases provide information that is relevant to the topic but not essential to the meaning of the sentence):
Correct: Dr. John Smith, an associate professor in the Department of Experimental Biology, is building a database of epigenetic changes in acute lymphoblastic leukemia. But: The Dr. John Smith who works in New York City must not be confused with the Dr. John Smith who was recently arrested for fraud. (The clauses beginning with “who” tell you which of two John Smiths is meant.) Correct: Many types of cancer, such as breast cancer and colon cancer, can be cured if diagnosed at an early stage. But: Only breast cancers that are diagnosed at an early stage can be cured. (The clause beginning with “that” indicates which cancers are potentially curable.) Use commas before and after a nonrestrictive appositive (an appositive renames or defines a noun; degrees, years in dates, and many geographical names are often treated like nonrestrictive appositives):
Correct: DNA methylation, the addition of a methyl group to one of the bases of DNA, is a fundamental part of gene expression.
Not: The diagnosis was Refsum’s disease or phytanic acid storage disease. (Without a comma, the sentence says that the patient had one of two diseases.) Correct: The diagnosis was Refsum’s disease, or phytanic acid storage disease. (With a comma, the sentence says the patient had Refsum’s disease, which is also known by another name. The word “or” is often used before nonrestrictive appositives.)
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