If you’ve all been waiting with bated breath for the continuation of my discussion of subordinate ideas and the dreaded comma, today’s your lucky day!
The next major topic in this arena is restrictive vs. non-restrictive clauses. The terminology sounds scary, but the distinction is simple. The subordinate ideas in question are extra. This information is not essential to understanding who or what is being spoken about (subject/object) or what is happening (main action). It’s the cherry on the sundae.
Restrictive clauses are groups of words that help define something (subject/object). We DO NOT separate these from the rest of the sentence using commas. THINK “THAT” (see note below).
Here’s an example.
The cat that chewed up my power cord clawed a hole in my air mattress.
In this sentence, the subject isn’t just any cat. The subject is the cat with a history of chewing. Now that the writer has defined the subject, readers understand that *that* cat also popped the air mattress. If the writer had used commas around “that chewed up my power cord,” readers wouldn’t have a clear idea about which cat is the subject (the subject would have been simply “the cat”).
Non-restrictive clauses are those offering extra information about an already defined person, place, or thing (subject/object). We DO separate these from the rest of the sentence using commas. THINK “WHICH” (see note below).
We use non-restrictive clauses when we’ve already defined the subject/object so well that readers know exactly what or who the subject/object is, but we want to include other information (unrelated to the main action).
Here’s an example.
The new teacher, who graduated from Yale University, teaches AP Language in addition to the regular high school literature courses.
In this case, “new teacher” suffices to describe the subject. He/she is the teacher who just started at that school. The action is teaching. The fact that *that* teacher graduated from Yale is not necessary to define him/her as a subject (to answer the question which teacher?) or necessary for the action (teaching), but the writer wanted to include it, so he/she placed the clause between two commas.
IMPORTANT NOTE
In American English, when “who” is not applicable (when we are not writing about humans), we use “that” for restrictive (defining) clauses and “which” for non-restrictive (extra information) clauses.
We also sometimes use “when” or “where” instead of “who,” “that,” or “which.”
Examples
The stadium where the Yankees played the Mets in 2005 is under construction.
Here “where the Yankees played the Mets in 2005” is necessary to tell readers which stadium the writer means.
I miss my childhood home, where I built my first treehouse.
Here “where I built my first treehouse” is extra information because readers understand that the home in question is the home where the writer spent his/her childhood.
Jane Austen wrote books in the early 19th century, when female authors struggled to find publishers.
The time when women couldn’t vote was particularly hard for women in the workplace.