Let’s go back to middle school English class for a moment. What is a conjunction? Conjunctions are parts of speech that join two words or phrases together. There are three types of conjunctions: coordinating, subordinating, and correlative. Coordinating conjunctions join two grammatically equal words or phrases. They’re usually remembered by the acronym FANBOYS: for, and, nor, but, or, yet, so. Subordinating conjunctions join a dependent clause to an independent clause. They make one phrase supplementary to the other. Correlative conjunctions function in the same way as coordinating conjunctions, but they’re always used in pairs (either/or, both/and, etc.). When connecting clauses, a comma is always used before a coordinating conjunction but never before a subordinating conjunction.¹

I was washing the dishes, and he was watching a movie.
I was washing the dishes while he was watching a movie.

The dog was sad, but the cat was happy.
The dog was sad that the cat was happy.

Coordinating conjunctions give both clauses equal weight, while subordinating conjunctions give more weight to the first clause. Similarly, subordinating conjunctions can be used to start a sentence, but coordinating conjunctions cannot.²

Correct: After I went to bed, the phone rang.
Incorrect: And the phone rang, I went to bed.

The word because is commonly taught to be a subordinating conjunction. It shows causation, automatically making one statement dependent on the other.

He went to bed because he was tired.
Because it’s cold, I put on a coat.

This is true. It is a subordinating conjunction. But it’s not just a subordinating conjunction. See, linguistic trends have made it into so much more than that. The word because can also function as a preposition, such as in the phrase “because physics.” Some consider because of to be a compound preposition, rather than a conjunction-preposition combo, but we’ll come back to that. Because as a preposition came about because the internet. Language and grammar are constantly evolving, and people are constantly introducing new ways to use words. When these catch on, we suddenly have a new rule of grammar.³

That being said, I am proposing another update to the colorful grammatical history of because: as well as a subordinating conjunction and a preposition, because is a coordinating conjunction.

While because often relegates one clause to a dependent role, it doesn’t always.

The boy is sick because he didn’t wear a coat.
The boy is sick, because the flu is bad this year.

The coordinating conjunction for has become largely archaic and has been replaced in modern writing with because. But what’s the difference?

Here’s some boring but necessary background for you: The traditional seven coordinating conjunctions can be split even further into four categories: cumulative, adversative, disjunctive, and illative. Cumulative conjunctions (and), also known as copulative or additive conjunctions, add words or phrases together. Adversative conjunctions (but, yet) express contrast between words or phrases. Disjunctive conjunctions (or, nor) present alternatives. Illative conjunctions (for, so), also known as final conjunctions—also the weird black sheep of the conjunction family—denote a phrase that is inferred from or a result of another phrase. Where cumulative and disjunctive conjunctions place equal emphasis on the clauses they are connecting (this and that; that or this), adversative and illative conjunctions place more emphasis on the secondary clause, as it would either be in direct contrast to the first (adversative) or the grounds for or consequence of the first (illative).

Illative conjunctions do not denote cause. Cause places emphasis on the first clause (the phrase that is being caused) using a subordinating conjunction.

He put on a coat because he was cold.

In this sentence, because functions as the subordinating conjunction everyone knows it to be. The first clause (he put on a coat) is emphasized over the second (he was cold), since the second is simply the cause of the first.

He put on a coat, for he was cold.

This sentence isn’t different, not really. But for is a coordinating conjunction. The distinction is tricky, largely archaic, and frankly a bit arbitrary, but here it is: for does not indicate causation but inferred consequence. The cold is not the cause of the man putting on a coat, but rather, putting on a coat is assumedly the consequence of the man being cold. Absurd, right? But take this example:

She is generous, for she helped me.

The first clause (she is generous) is an inference based on the second clause (she helped me). The sentence could be rewritten: She is a generous person, which I know because she helped me. If, however, you one-for-one replace the coordinating for with the subordinating because, it changes the meaning of the sentence.

She is generous because she helped me.

No longer is she assumed to be a generous person with the single instance of her helping generosity being taken as proof, her generosity is relegated to the one instance in which she offered help. I’m splitting hairs, I know. But bear with me. For and because are no longer perfectly interchangeable in this example, as the coordinating conjunction gives equal weight to the clauses and the subordinating conjunction does not. Now take this sentence:

She is generous, because she helped me.

Most editors and grammarians would tell you that this sentence is incorrect, that you should never put a comma before a subordinating conjunction. And you shouldn’t. But the sentence makes sense, does it not? The comma returns the sentence to its former meaning, placing equal emphasis on both clauses. (Or even, like other illative conjunctions, stronger emphasis on the second clause.) Why? Because because has evolved past being a lowly Latin-based, by-the-cause-of subordinating conjunction. It has replaced for in the modern lexicon and functions as not only a subordinating conjunction and a preposition, but also as a coordinating conjunction.

So. If you accept my premise that because is both a subordinating and coordinating conjunction, how do you tell the difference between the two? Does your sentence warrant a comma or not?

Well, that largely depends on what you want your sentence to say. If you’re using because simply to say that this happened because that happened, then you’re using a subordinating conjunction. No comma. If you’re using because to imply that the first statement is a logical conclusion that can be drawn from the second, you’re using a coordinating conjunction. Comma. If you’re unsure, you’re probably using a subordinating conjunction. Default to the traditional rules of grammar. No comma. Grammar exists to make written language coherent from one person to the next, so if adding or deleting a comma is going to change the meaning of your sentence, go with the structure that says what you want it to. If you don’t really see the difference or don’t care, go with the default rules you were taught.

Whether or not you accept my premise that because is a coordinating conjunction—though there is no denying that’s how the word is popularly used—it’s important to point out that nobody perfectly agrees on the exact rules of grammar. Remember how some people believe that because of is a compound preposition? They’re not wrong, it’s just another way of looking at something. It barely changes functionality, if at all. Mostly it just changes how sentences are diagrammed, which can be done many different ways anyway thanks to language ambiguity. Language is constantly evolving, and the rules of grammar are, at their base, a way of making written language decipherable. They change as language changes. And while that doesn’t mean you can throw all the rules of grammar out the window because you don’t feel like learning them, it means that there’s a little bit of wiggle room in the specifics.

But either way, for the love of all that’s good in this world, don’t add a comma just because you take a breath.

¹Commas before subordinating conjunctions are sometimes used for clarity, especially in the case of negated statements or extreme contrast, but as a rule of thumb, putting a comma before one is incorrect.

²While there’s great debate whether sentences in formal writing can be started with coordinating conjunctions, it’s always done with only one clause, never two as in the example.

³This does not mean that you can excuse your terrible grammar and spelling with “because language evolution.” Language still has to have structure, otherwise no one would be able to understand each other.