We’ve looked at dependent and independent clauses in a previous Building Blocks. Now let’s look at [tag]relative clauses[/tag].
Specifically, let’s look at restrictive and nonrestrictive clauses, also known as essential and nonessential clauses. To define, a restrictive (essential) clause adds important information to a sentence and cannot be eliminated without changing the meaning of the sentence. A nonrestrictive (nonessential) clause, however, adds parenthetical information that can easily be eliminated without changing the meaning of the sentence.
Let’s look at some examples:
The man who is wearing the red hat just murdered the bank teller.
Here, the clause "who is wearing the red hat" specifically limits the murderer to that one man with the chapeau. You cannot remove this clause without changing the meaning of the sentence.
The man, who is wearing a red hat, asked for a glass of water with his meal.
Here you have the same clause, but it’s nonrestrictive. The meaning of the sentence is solely contained in the act of asking for a glass of water. The red hat has no bearing on the meaning; it’s extra, parenthetical information. (I admit, however, that this is not the greatest example since you can remove the commas and make this a restrictive clause.)
Notice that the commas signify that the clause in question is nonessential or nonrestrictive. You never use commas with a restrictive clause, and the presence of the commas signals the reader that the information is not essential.
The person who reads and masters this will better understand relative clauses.