Often you’ll see a GMAT sentence correction question that features a word or phrase set apart from the rest of the sentence with commas. Take the following example:

The artist constructs his sculptures using found objects, driftwood or pieces of trash, that wash up on the beach.

The phrase driftwood or pieces of trash describe the noun that precedes it, objects. It identifies what exactly the objects are. The technical term for this kind of modifier is a “non-restrictive appositive,” and GMAC loves to write sentence correction questions that feature them.

Don’t worry about the terminology; just remember what non-restrictive appositives look like and do: They’re words or phrases set aside with commas that provide more information about a noun in the sentence. Keep the following rules in mind and you’ll be able to spot non-restrictive appositive errors whenever they appear in sentence correction questions.

Rule 1: Appositives need to be close to the noun they modify

Appositives follow the golden rule of modifiers, which states that modifiers need to be placed as close as possible to what they modify in a sentence. Failure to follow this rule leads to unclear or illogical sentences. For example, moving the appositive in the sentence above changes which noun it modifies.

The artist constructs his sculptures, driftwood or pieces of trash, using found objects that wash up on the beach.

Driftwood or pieces of trash now describes sculptures instead of found objects, and the sentence no longer makes much sense. When you see an appositive marked by commas in a question, make sure it’s modifying the intended noun.

Rule 2: The sentence must still make sense if the appositive is removed

This is really a two-part rule because by sense we mean both grammatical sense, i.e. that grammatical rules are followed, and logical sense. In the first case, non-restrictive appositive modifiers are often used to hide other grammatical errors in the sentence.

For example, the GMAT will sometimes insert a lengthy appositive between a sentence’s subject and verb in order to obscure a subject-verb agreement error. Removing the appositive from the sentence helps reveal any errors.

Wiffle ball and kickball, two games popular with most American children, is not common outside of the U.S.

Without the appositive, the subject-verb agreement error becomes more apparent.

Wiffle ball and kickball is not common outside of the U.S.

Sentences must also make logical sense if a non-restrictive appositive is removed. To understand what we mean by logical sense, it helps to understand the term “non-restrictive.” If a modifier, such as an appositive, is restrictive, its use in the sentence is not optional. It must be included because it contains information essential to the sentence’s meaning. You’re restricted from removing the modifier because otherwise the sentence’s meaning would change.

Non-restrictive modifiers on the other hand can be removed from a sentence, and the meaning of the sentence will not change. That’s because non-restrictive modifiers provide additional information that’s not essential to the sentence’s meaning. The example below contains what appears to be a non-restrictive appositive since a phrase that describes a noun is contained between commas. However, this sentence would be incorrect on the GMAT.

Rectangles, with four equal sides, are also squares.

Removing the appositive would result in Rectangles are also squares, a grammatically correct but illogical sentence. With four equal sides is restrictive — it’s essential to the meaning of the sentence — so it should not be contained in commas.

This may seem like a lot of information to remember for dealing with such simple and common structures, but if you remember the two rules, spotting errors won’t be challenging. When you see an appositive in a question, go through the rules, asking What noun is the appositive modifying? and Does the sentence make sense grammatically and logically if the appositive is removed?

Keep appositive attitude, and you’ll do fine.