To show you how commas work, I’m going to compare two different sentences and I will underline the adjective clause in each sentence to make sure that you know what I’m talking about.
1. Never buy a car that has over 200,000 miles on it.
2. My truck, which is a Nissan, has over 60,000 miles on it.
Okay, to teach you the key concept here for deciding on commas (restrictive and non-restrictive), I’m going to ask you to cross out the adjective clause in each sentence.
Let’s start with #1.
Does that make a difference in the meaning of that sentence—whether it stays in or comes out?
You bet it does! If you take out the adjective clause in the first sentence, you essentially kill the sentence. This is because the clause is necessary to keep the meaning of the sentence. Therefore we say that the clause is “restrictive”.
In sentence #1 we’re not saying “never buy cars”—that’s crazy! What we’re doing instead is restricting the meaning of cars NOT TO BUY, to “cars that have over 200,000 miles on them.” Think about it; that makes sense. Who wants to take a chance on an old beater of a car?
The important thing is this: restrictive or necessary or essential adjective clauses DO NOT GET COMMAS.
Look, I understand that there seems to be a bit of a disconnect here. Maybe you hear it this way:
So I’m going to give you a way of understanding this in terms of grammar.
Can you imagine seeing this?
*I, am Mr. Lund.
Of course not! The subject and the verb are the meat and potatoes, the ying and the yang, of a sentence. We would never see a comma between the subject and the verb.
So that is the principle here—DON’T USE COMMAS TO SET OFF ADJECTIVE CLAUSES THAT ARE ESSENTIAL TO THE MEANING OF THE SENTENCE.
Now you can probably guess where we do use commas. Look at sentence #2. What happens if you take out the clause? Not a whole lot, right?
Not much change this time is there? This clause (“which is a Nissan”) basically just gives us EXTRA information. It’s interesting but not essential. What’s important in this sentence is not what kind of truck it is but how many miles it has on it. Well that means that for non-essential or non-restrictive adjective clauses we use commas to set them off from the rest of the sentence.
Here is the mastery rule and the memory sentences that apply to the adjective clauses.
This lesson is covered in much more detail in Chapter 4, Lesson 6 of Loving Grammar: Mr. Lund’s Guide to Professional Clamdigging™.
Go to www.lovinggrammar.com for more information.