GrammarPhile Blog

When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.

In the spirit of Independence Day, freedom and such, this first sentence of the Declaration of Independence provides a springboard for a timely discussion of restrictive versus nonrestrictive clauses.

Restrictive Clauses

A restrictive clause (also called an essential clause) provides essential information about the noun that precedes it. In other words, the sentence wouldn’t be grammatically or logically complete without it. The restrictive clauses in the following examples are highlighted in red:

Southwest is the only airline that doesn’t charge extra for baggage.

Captain Hook is afraid of the crocodile that has a ticking alarm clock in its stomach.

Tom’s sister Sally works in retail.

Without the red text, the remaining fragments may be ambiguous or make no sense.

Because the text in a restrictive clause is essential to understanding the sentence’s meaning, restrictive clauses are not set off by commas (which would suggest the additional information is supplemental). In the last example above, the fact that Sally is not set off by commas implies that Tom has more than one sister and that we need the identifier Sally to clarify which sister the sentence is referencing. Compare the similar example in the next section to understand when Sally should be set off by commas.

Nonrestrictive Clauses

Nonrestrictive (nonessential) clauses are supplemental information — nice to know but not needed for the sentence to be clear or make sense. These clauses typically are set off by commas. Variations of the above examples follow, with the nonrestrictive clauses highlighted in blue:

Southwest, which serves more than 100 million customers annually, is the only airline that doesn’t charge extra for baggage.

Captain Hook is afraid of the crocodile that pursues him relentlessly, which has a ticking alarm clock in its stomach.

Tom’s youngest sister, Sally, works in retail.

Even with the blue text removed, the above sentences are clear and make sense.

That vs. Which

Understanding the distinction between restrictive and nonrestrictive clauses can help in figuring out whether to use that or which in a sentence — one of the most common problems we encounter in documents that come through ProofreadNOW.com.

Most often, which is erroneously used for that, as shown above in red in the Declaration of Independence passage. Its usage there is understandable, given that the two terms are largely interchangeable in British English and that British English is evidenced throughout that document: offences, neighbouring, amongst, endeavoured, etc.

In American English, however, the strong preference is to distinguish the two terms according to the following guidelines:

• If the sentence isn’t clear or doesn’t make sense without the clause, the clause is restrictive: Use THAT without a comma.
• If the sentence can stand alone when the clause is omitted, the clause is nonrestrictive: Use WHICH and/or commas to set off the clause.
• If which starts a clause and is not preceded by a comma, it likely should be THAT.

Here’s hoping your Fourth of July (or, if you’re outside the U.S., your weekend) is free from the tyranny of grammar oppressors everywhere.