“Clause” is one of those terms that gets thrown around a lot in discussion about grammar. It’s one of those words that students often hear but whose meaning they tend not to be 100% sure of.

It’s certainly possible to study for the SAT®/ACT®/GMAT® without knowing the exact definition of a clause, but understanding what clauses are and how they work can make things a whole lot easier.

So here goes:

A clause is a phrase that contains both a subject and a verb.

That might sound suspiciously like the definition of a sentence, and in fact, some clauses are sentences. But not all.

There are two main types of clauses:

I. Independent Clause

Independent clauses can stand on their own as complete sentences.

Examples (subject underlined, verb in bold):

• Halley’s Comet was the first comet observed in detail by spacecraft.
• Growing flowers indoors is fairly simple.
• They invented their own language.

Independent clauses must be separated from one another by periods or semicolons, not commas:

Incorrect: Growing flowers indoors is fairly simple, anyone can do it.
Correct: Growing flowers indoors is fairly simple. Anyone can do it.
Correct: Growing flowers indoors is fairly simple; anyone can do it.

II. Dependent Clause

Dependent clauses cannot stand on their own as complete sentences. (To simplify things, you can also think of them as fragments.)

For test-prep purposes, there are two main types of dependent clauses you need to be familiar with.

You do not need to memorize all of the terminology associated with them, but you do need to be able to reliably recognize when a clause is dependent because that factor often affects how it can be punctuated.

A. Subordinating conjunction

Common subordinating conjunctions include because, when, if, until, (al)though, while, and whereas.

Examples (subject underlined, verb in bold):

• Because Halley’s Comet was the first comet observed in detail by spacecraft
• While growing flowers indoors is fairly simple
• Although astronomy and astrology share a common set of origins

This type of dependent clause can be placed either before or after an independent clause to form a complete sentence.

Note that unlike two independent clauses, a dependent clause and an independent clause can be separated from each other by only a comma.

Before: Although they share a common set of origins, astronomy and astrology are now regarded as two entirely separate fields.
After: Astronomy and astrology are now regarded as two entirely separate fields, although they share a common set of origins.

Note also that a dependent clause begun by a subordinating conjunction must contain a conjugated verb — it cannot contain only an -ing word. (The GMAT in particular is fond of testing this rule, although the SAT and ACT test versions of it as well.)

Incorrect: Although sharing a common set of origins, astronomy and astrology are now regarded as two entirely separate fields. Correct: Although they share a common set of origins, astronomy and astrology are now regarded as two entirely separate fields.

B. Relative clause: who(se), which, or that

Examples (subject underlined, verb in bold):

• who invented their own language
• whose first novel was published in 1995
• that grows in shallow water

This type of dependent clause can be inserted into an existing sentence to form an essential or non-essential clause.

Essential clauses are set of by that and are central to the meaning of a sentence. These clauses are not surrounded by commas:

Sentence: The sea cucumber has an oval body about 3 inches long.
Dependent clause: that grows in shallow water off the North American coast
Sentence: The sea cucumber that grows in shallow water off the North American coast has an oval body about 3 inches long.

Non-essential clauses are set off by which or who(se)*. These clauses are surrounded by commas.

Sentence: Twins Grace and Virginia Kennedy spoke little English until late in their childhoods.
Dependent clause: who invented their own language
Combined: Twins Grace and Virginia Kennedy, who invented their own language, spoke little English until late in their childhoods.

Notice that in the above example, the verb invented “belongs” to who, the subject of the dependent clause, rather than to the main subject of the sentence, Twins Grace and Virginia Kennedy.

(*Note: in some instances, clauses begun by who can be essential. See here and here for more information).