Across stylebooks, rules about the comma are some of the most frequent causes of debate. Once, I received an interesting question about using a comma before the word but. Here’s what the reader asked:
Is there a rule that a comma may only be used before but if what follows is a complete thought, such as an independent clause?
Let’s dive in.
But is a member of several parts of speech, or word classes. It can be a conjunction, preposition, adverb, or noun. Commas have many uses; Amy Einsohn identifies 15 different uses in The Copyeditor’s Handbook. Despite that, but is preceded by a comma only when it acts as a conjunction.
As the question notes, but is preceded by a comma when it acts as a coordinating conjunction for two independent clauses:
It may have been the site of Lincoln’s birthplace, but it wasn’t Lincoln’s cabin.
Christian Science Monitor (2005)
However, this combination also occurs when but joins an independent clause to a dependent clause if the clause is nonrestrictive:
A model of language acquisition with one dimension is better than one with two, but only to the point where the power to predict and explain phenomena is lost.
Journal of Speech, Language & Hearing Research (April 2012)
When placing a comma before but, evaluate the entire sentence first. If you’re joining two independent clauses or an independent clause and a dependent clause, the comma should be there. Remember, though, when it comes to writing and editing, there is no one universal style guide and no hard-and-fast rule for using a comma. Reference the style guide that you are using for the project and, whatever you decide, maintain consistency throughout the work.
Read more about independent clauses in “Punctuation Point: Joining Independent Clauses.”
