What Is a Conjunction?

A conjunction joins — but how it joins carries real consequences for punctuation, emphasis, and whether your sentence is grammatical at all. The word class is small and the everyday members are familiar, yet more comma errors trace back to conjunctions than to any other part of speech. This article sorts them into their three classes and shows where each one earns, or forbids, a comma.

What this covers. A working definition; the three classes (coordinating, subordinating, correlative); the comma rules each one triggers; and two myths worth retiring — the ban on sentence-initial and/but, and the fear of the subordinate clause.

The definition

A conjunction joins words, phrases, or clauses into a single grammatical unit. The joining can be between equals — two nouns, two verbs, two independent clauses — or between unequals, where one clause is made grammatically dependent on another. That equal-versus-unequal distinction is the whole game, because it determines the class, and the class determines the punctuation.

The three classes

Coordinating conjunctions join equals: the seven FANBOYS — for, and, nor, but, or, yet, so. When they join two independent clauses (each of which could stand alone), the house rule is a comma before the conjunction: The figures were late, so the meeting slipped. When they merely join two words or phrases, no comma: revenue and costs, slowly but deliberately. The detail — including the correlative pairs — lives in coordinating & correlative conjunctions.

Subordinating conjunctions join unequals: because, although, since, while, if, when, before, after, unless, whereas. They open a subordinate clause that cannot stand alone and depends on the main clause for sense. Position drives the comma: lead with the subordinate clause and you need one — Because the vendor slipped, we rebased the plan; put it second and you usually don't — We rebased the plan because the vendor slipped. The set is covered in subordinating conjunctions.

Correlative conjunctions work in matched pairs: either…or, neither…nor, both…and, not only…but also, whether…or. Their one demand is parallelism — the structure after the first half must mirror the structure after the second: not only drafted the clause but also negotiated it (verb phrase to verb phrase), never not only drafted the clause but also the negotiation.

Two myths

Starting a sentence with and or but is fine. It's a stylistic choice, not an error, and every major usage guide agrees. Used sparingly it adds punch; overused it grates — the same as any device. The Victorian schoolroom ban has no grammatical basis.

A subordinate clause is a tool, not a hazard. The instinct to avoid because, although, and while produces flat, choppy prose. Subordination is precisely how you signal which idea is primary and which is supporting — the reader should not have to guess.

Common Mistake: the comma splice, which a conjunction cures. The vendor slipped, we rebased the plan joins two independent clauses with only a comma — an error. Add a coordinating conjunction (…slipped, so we rebased…), subordinate one clause (Because the vendor slipped, we rebased…), or use a semicolon. Never the bare comma. See FANBOYS and comma splices.

Check Your Understanding

1. Name the class: We shipped late because QA found a regression.

2. Punctuate: The estimate was optimistic ___ the deadline held. (join two independent clauses with yet)

3. Fix the parallelism: She is not only fluent in French but also she speaks Mandarin.

4. True or false: beginning a report sentence with But is a grammatical error.

Where next

• Coordinating & Correlative Conjunctions — the FANBOYS and the matched pairs, in full.
• Subordinating Conjunctions — dependent clauses and the comma that leads them.
• Conjunctive Adverbs vs Conjunctions — why however and therefore aren't conjunctions and punctuate differently.
• → The Parts of Speech — how conjunctions relate to the other seven word classes.