More on restrictive clauses and commas

Just copying and pasting this from that old style guide I wrote. It might be useful to see some more examples, so here they are--

A nonessential (or nonrestrictive) subordinate clause is one which is not necessary to the meaning of the sentence but merely adds an additional idea. These clauses take commas.

If the essential meaning of the sentence changes, then the clause does not take commas. (Essential phrases and clauses do not take commas.)

Test by removing the clause to see if the meaning of the sentence changes.

Example: Jane Smith is the only cheerleader who didn’t get pregnant her senior year.

If you remove the clause, you get:

Jane Smith is the only cheerleader.

The meaning of the sentence is not the same. Therefore, the clause is essential and does not take commas.

Compare to: Jane Smith, who was the only cheerleader not to get pregnant her senior year, accepted a scholarship at Harvard.

If you remove the clause, you get:

Jane Smith accepted a scholarship at Harvard.

The meaning is the same. The extra clause merely adds new information. The clause is therefore nonessential and requires commas.

The same test and rules applies to essential and nonessential participial phrases.

Another example:

NONESSENTIAL: My sister, who works for the bank, drives a company car. (I have only one sister, and she happens to work for the bank.)

ESSENTIAL: My sister who works for the bank drives a company car. (I have more than one sister, and I am specifically referring to the one who works for the bank.)

Theresa