Commas are among the most misused punctuation marks in English. They have specific rules for lists, clauses, introductory phrases, and more. Both missing commas and unnecessary commas change the rhythm and sometimes the meaning of a sentence.
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Commas in Lists
Use commas to separate items in a list of three or more. "She bought apples, oranges, and bananas." The comma before "and" at the end of a list is called the Oxford (or serial) comma. In British English it is often omitted; in American English it is usually kept. When the last two items could be confused without it, the Oxford comma is essential: "I'd like to thank my parents, Einstein, and Newton" vs. "my parents, Einstein and Newton" (which implies your parents are Einstein and Newton).
Commas with Subordinate & Relative Clauses
Use a comma after an introductory subordinate clause that comes before the main clause: "Although it was late, she stayed to finish the work." No comma is needed when the subordinate clause comes second: "She stayed to finish the work although it was late." For non-defining relative clauses, commas are required on both sides: "My brother, who lives in Berlin, is visiting next week."
Commas Joining Independent Clauses
Use a comma + coordinating conjunction (for, and, nor, but, or, yet, so — FANBOYS) to join two independent clauses: "She studied hard, and she passed." Without the conjunction, a comma alone creates a comma splice error: "She studied hard, she passed." ✗ — use a period, semicolon, or add a conjunction. A comma splice is one of the most common writing errors.