Non-essential clauses — also known as “parenthetical” or “non-restrictive” clauses — are one of the most important concepts tested on both SAT Writing and ACT English. They are used to provide extra information about or description of a noun, usually the subject, and they have several important characteristics:
1) They are always surrounded by commas (that means comma before and after)
2) They can be removed from a sentence without affecting its essential meaning
3) They are usually — but not exclusively — followed by verbs
With non-essential clause: My history textbook, which is 500 pages long and weighs almost 10 pounds, is the most boring book I have ever read.
If we cross out the non-essential clauses, we are left with:
Without non-essential clause: My history textbook…is the most boring book I have ever read.
The sentence still makes perfect sense.
One important thing to know about non-essential clauses is that they often begin with either “who” or “which,” as in the above sentence. It is not, however, necessary that they begin with one of these words. When they begin with something else, they are known as appositives (I’m simplifying a bit here, but this is the gist of it). You do not need to be able to identify appositives by name, but you do need to be able to recognize that the construction is correct, even if it may sound funny to you.
Correct: My history textbook, a 500 page monster, is the most boring book I have ever had to read.
The presence of two commas within a sentence does not necessarily indicate the presence of a non-essential clause, however. There are many other reasons why two commas can appear in a sentence. If you’re not sure, simply cross out the information between the commas, and check to see that the remaining sentence still makes sense.
Not a non-essential clause: Yesterday, I tried to read a chapter of my history textbook, but it was so boring that I fell asleep.
In the above sentence, there are two commas, but if we cross out the information between them, we are left with:
Crossed-out: Yesterday…but it was so boring that I fell asleep.
Clearly this does not make sense! So the information between the commas is not a non-essential clause.