Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021
Use a comma before a coordinating conjunction joining independent clauses
The comma
Punctuation
✵ 33 The comma
✵ 34 Unnecessary commas
✵ 35 The semicolon
✵ 36 The colon
✵ 37 The apostrophe
✵ 38 Quotation marks
✵ 39 End punctuation
✵ 40 Other punctuation marks
33The comma
The comma was invented to help readers. Without it, sentence parts can collide into one another unexpectedly, causing misreadings.
CONFUSING |
✵ If you cook Elmer will do the dishes. |
CONFUSING |
✵ While we were eating a rattlesnake approached our campsite. |
Add commas in the logical places (after cook and eating), and suddenly all is clear. No longer is Elmer being cooked, the rattlesnake being eaten.
Various rules have evolved to prevent such misreadings and to speed readers along through complex grammatical structures. Those rules are detailed in this section. (Section 34 explains when not to use commas.)
33a Use a comma before a coordinating conjunction joining independent clauses.
When a coordinating conjunction connects two or more independent clauses — word groups that could stand alone as separate sentences — a comma must precede the conjunction. There are seven coordinating conjunctions in English: and, but, or, nor, for, so, and yet.
A comma tells readers that one independent clause has come to a close and that another is about to begin.
EXCEPTION: If the two independent clauses are short and there is no danger of misreading, the comma may be omitted: The plane took off and we were on our way.
NOTE: Do not use a comma with a coordinating conjunction that joins only two words, phrases, or subordinate clauses. (See 34a.)
