Non-essential clauses — also known as “parenthetical” or “non-restrictive” clauses — are among the most important concepts tested on both SAT Writing and ACT English. They are used to provide additional information about nouns, usually the subject, and they have several important characteristics:

1) They are most often surrounded by commas (one before, one after), but they can also be surrounded by dashes or parentheses.

2) They can be removed from a sentence without affecting its essential meaning.

3) They are usually followed by verbs.

Let’s start with the following base sentence:

Correct: The Olmec culture, which flourished in the modern-day Mexican states of Veracruz and Tabasco, was among the earliest complex civilizations.

Correct: The Olmec culture which flourished in the modern-day Mexican states of Veracruz and Tabasco was among the earliest complex civilizations.

Correct: The Olmec culture (which flourished in the modern-day Mexican states of Veracruz and Tabasco) was among the earliest complex civilizations.

If we cross out the non-essential clauses, the sentence that remains still makes perfect sense:

Crossed out: The Olmec culture…was among the earliest complex civilizations.

The sentence still makes perfect sense.

Appositives

One important thing to know about non-essential clauses is that they often begin with either who or which, as in the above sentence. They can, however, also begin with nouns, in which case they are known as appositives. I’m simplifying a bit here, but this is the gist of it. You do not need to be able to identify appositives by name, but you do need to be able to recognize that the construction is correct, even if it may sound funny to you.

Correct: The Olmec culture, a culture that flourished in the modern-day Mexican states of Veracruz and Tabasco, was among the earliest complex civilizations.

Making Sure It’s Really A Non-Essential Clause

Some people have difficulty determining when the presence of two commas actually indicates a clause that can be crossed out (if this applies to you, keep reading; if not, you can ignore the rest of this post). There are many other scenarios when two commas can appear in a sentence and in fact nothing can be crossed out.

No non-essential clause: Beginning around 1500 BC, the Olmec culture flourished along the Gulf Coast of Mexico, primarily in the modern-day states of Veracruz and Tabasco.

In the above sentence, there are two commas, but if we cross out the information between them, we are left with:

Crossed-out: Beginning around 1500 BC…primarily in the modern-day states of Veracruz and Tabasco.

Clearly this does not make sense! The information between the commas is therefore not a non-essential clause.

One of the most common errors involving non-essential clauses involves removing one of the commas.

Incorrect:  The Olmec culture which flourished in the modern-day Mexican states of Veracruz and Tabasco, was among the earliest complex civilizations. (No comma before the non-essential clause)

Incorrect: The Olmec culture which flourished in the modern-day Mexican states of Veracruz and Tabasco was among the earliest complex civilizations. (No comma after the non-essential clause)

One thing to be aware of is that non-essential elements can be both quite long and very, very short. The important thing to remember is that regardless of length, the rule stays the same: if you can take the word or phrase out of the sentence and the the sentence still makes sense, you need both commas. No exceptions.

Correct: The Olmec culture began to flourish around 1500 BC. It is, therefore, among the earliest complex civilizations.

Correct: The Olmec culture began to flourish around 1500 BC. It is, historians believe, among the earliest complex civilizations.

Mismatched Punctuation

Both the SAT and the ACT test punctuation involving non-essential clauses. The most important rule is that the same type of punctuation should be used to mark both the beginning and the end of a non-essential clause: commas must be matched with commas, dashes with dashes, and parentheses with parentheses. Any construction that involves mixing and matching is incorrect.

In most cases, dashes and commas will be mismatched with one another:

Incorrect: The Olmec culture, which flourished in the modern-day Mexican states of Veracruz and Tabasco — was among the earliest complex civilizations.

Incorrect: The Olmec culture  which flourished in the modern-day Mexican states of Veracruz and Tabasco, was among the earliest complex civilizations.

Note that catching these errors often requires you to read either backwards or forwards within a sentence. If the underlined portion is at the beginning of the sentence, you will have to jump ahead to check the punctuation at the end of the non-essential clause; and if the underlined portion is at the end of the sentence, you will have to backtrack to check the punctuation at the beginning of the non-essential clause. That is why you must consider each underlined section in context and not only focus on the specific words in question.

Additional Errors 

1) Subject-Verb Agreement Errors

Incorrect: The Olmec culture, which flourished in the modern-day Mexican states of Veracruz and Tabasco, were among the earliest complex civilizations.

Correct: The Olmec culture, which flourished in the modern-day Mexican states of Veracruz and Tabasco, was among the earliest complex civilizations.

Occasionally, however, the error may appear within the non-essential clause:

Incorrect: The Olmec culture, which were among the earliest complex civilizations, flourished in the modern-day Mexican states of Veracruz and Tabasco.

Correct: The Olmec culture, which were among the earliest complex civilizations, flourished in the modern-day Mexican states of Veracruz and Tabasco.

2) Fragments

Incorrect: The Olmec culture, which flourished in the modern-day Mexican states of Veracruz and Tabasco, and it was among the earliest complex civilizations.

To fix the sentence, you must start, as always, by crossing out the non-essential clause in order to see what you’re truly dealing with:

Incorrect: The Olmec culture, which flourished in the modern-day Mexican states of Veracruz and Tabasco, and it was among the earliest complex civilizations.

Once you’ve reduced the sentence, you simply cross out everything before the verb:

Correct: The Olmec culture…and it was among the earliest complex civilizations.

The correct answer choice will pretty much always be the one that contains a comma immediately followed by a verb.

You could also encounter a question that asks you to fix the beginning of a non-essential clause. In order to do so, you must be able to read ahead in the sentence and recognize that a comma can mark the end of a non-essential clause.

Incorrect: The Olmec culture flourished in the modern-day Mexican states of Veracruz and Tabasco, was among the earliest complex civilizations.

If you just read the beginning, the sentence seems fine. But if you read the whole thing, something is clearly off. The key is to recognize that the comma before was can mark the end of a non-essential clause. To fix the sentence, you can therefore begin the non-essential clause after culture.

Correct: The Olmec culture, which flourished in the modern-day Mexican states of Veracruz and Tabasco, was among the earliest complex civilizations.