Nonrestrictive elements are word groups that do not limit, or restrict, the meaning of the noun or pronoun they modify. Setting nonrestrictive elements off with commas shows your readers that the information is not essential to the meaning of the sentence. Restrictive elements, on the other hand, are essential to the meaning and should not be set off with commas. The same sentence may mean different things with and without the commas:
The bus drivers rejecting the management offer remained on strike.
The bus drivers, rejecting the management offer, remained on strike.
The first sentence says that only some bus drivers, the ones rejecting the offer, remained on strike. The second says that all the drivers did.
Since the decision to include or omit commas affects how readers interpret your sentence, you should think especially carefully about what you mean and then use commas (or omit them) accordingly.
| RESTRICTIVE | Drivers who have been convicted of drunken driving should lose their licenses. |
In the preceding sentence, the clause who have been convicted of drunken driving is essential because it explains that only drivers who have been convicted of drunken driving should lose their licenses. Therefore, it is not set off with commas.
| NONRESTRICTIVE | The two drivers involved in the accident, who have been convicted of drunken driving, should lose their licenses. |
In this sentence, however, the clause who have been convicted of drunken driving is not essential to the meaning because it does not limit what it modifies, The two drivers involved in the accident, but merely provides additional information about the drivers. Therefore, the clause is set off with commas.
To decide whether an element is restrictive or nonrestrictive, mentally delete the element, and see if the deletion changes the meaning of the rest of the sentence. If the deletion does change the meaning, you should probably not set the element off with commas. If it does not change the meaning, the element probably requires commas.