A phrase is a group of words that lacks a subject (an actor) and a verb (an action):
- after the market correction (prepositional phrase)
- the clever stock traders (noun phrase)
- were ready to buy the dip (verb phrase).
An Essential Phrase is a phrase that contains the information needed to complete the meaning of the sentence.
A Non-Essential Phrase is a phrase that contains information that isn’t needed in the sentence for the sentence to retain its meaning.
Synonymous Terms: The terms restrictive or non-restrictive information are synonymous to Essential Phrase or Non-Essential Phrase.
Being able to distinguish between Essential Phrase and Non-Essential Phrase is critical to effective comma usage.
Commas are used to set off, or highlight, non-essential information in a sentence. Essentially, if you could cross out the information and the sentence would still mean the same thing, then you should put a comma before and after it (to “set off” here means to place a comma before and after a group of words, thereby visually highlighting the words and allowing the reader to pause to consider the interesting, but not critical, information).
James, who is a part-time aviator, loves to tinker with machines of all kinds.
In this sentence, the subject is James, and the verb is loves. Readers do not need to know that James flies planes occasionally in order to understand that he loves to tinker with machines. The words who is a part-time aviator are literally not essential to the sentence. It works just fine without them:
James loves to tinker with machines of all kinds.
Therefore, those words are non-essential to the sentence’s meaning. The reader could cross them out, and the sentence would not change in meaning. Commas illustrate non-essential information by appearing before and after the non-essential information.
Let’s consider another example:
People who study hard make good students.
If you put commas around who study hard, you are saying that that information is non-essential.
People, who study hard, make good students.
This latter example suggests all people make good students. This is a problem, since this sentence does not mean the same thing as the first sentence. In the first one, the students who study hard are good students; in the second one, people in general make good students–and that is not always true! Therefore, the phrase who study hard is essential information to the meaning of this sentence. It cannot get commas around it without significantly changing the meaning!
See Use Commas Around Nonrestrictive Parenthetical Elements at Commas for a more extended discussion of restrictive vs nonrestrictive information.