When to Use a Comma Before “But”

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 Ein...