Even good writers make grammatical mistakes.
While content writers should have a grip on everyday grammar such as their versus they’re, some usage rules aren’t so black and white. That and which are often used interchangeably, but there is a definitive difference between them. Here’s an example:
Bob limped on his right foot that was injured.
Bob limped on his right foot, which was injured.
While both sentences are technically correct, “Bob limped on his right foot that was injured” is the better choice. Usage of that and which depends on whether the noun (foot) is followed by a restrictive or non-restrictive relative clause.
A restrictive clause (preceded by that and no comma) is information that is critical to the meaning of the sentence. A non-restrictive relative clause (preceded by which and a comma) provides information that is non-essential for meaning.
“That was injured” is a restrictive clause, because it allows us to understand why Bob is limping. If the sentence read, “Bob limped on his right foot, which was injured,” the use of which becomes appropriate, since “Bob limped on his right foot” conveys enough information to make sense.
Restrictive clauses are generally introduced by that and whose. Non-restrictive relative clauses are typically introduced by which, whose, who or whom. Incorrect usage of that and which can even subtly change a sentence’s meaning:
The dog, which was barking furiously, scared Tim.
The dog that was barking furiously scared Tim.
In the first sentence, which contains a non-restrictive relative clause, a dog scared Tim. In the second, which features a restrictive clause, it was the dog that was barking that scared Tim–not just any dog. While this explanation may be simple, it provides a good rule of thumb.
About the author
Tracey S is a former trade editor (editorial director, Customer Interaction Solutions magazine) turned freelance business content writer. She has written material (both bylined articles and ghost-written material or staff editorial) for a wide variety of clients, including those offering content in business software, telecommunications services and equipment, manufacturing and industrial design, aerospace, 3D printing and additive manufacturing, materials science, smart grid/smart city initiatives and green energy. Her background is in marketing, which dovetails nicely with content creation, search engine optimization and sales/marketing enablement.