KU Style Guide
These terms are used in this book instead of restrictive clause and nonrestrictive clause to convey the distinction between the two in a more easily remembered manner.
Both types of clauses provide additional information about a word or phrase in the sentence.
The difference between them is that the essential clause cannot be eliminated without changing the meaning of the sentence — it so restricts the meaning of the word or phrase that its absence would lead to a substantially different interpretation of what the author meant.
The nonessential clause, however, can be eliminated without altering the basic meaning of the sentence — it does not restrict the meaning so significantly that its absence would radically alter the author’s thought.
Punctuation: An essential clause must not be set off from the rest of a sentence by commas. A nonessential clause must be set off by commas.
The presence or absence of commas provides the reader with critical information about the writer’s intended meaning. Note the following examples:
— Reporters who do not read the Stylebook should not criticize their editors. (The writer is saying that only one class of reporters, those who do not read the Stylebook, should not criticize their editors. If the who … Stylebook phrase were deleted, the meaning of the sentence would be changed substantially.)
— Reporters, who do not read the Stylebook, should not criticize their editors. (The writer is saying that all reporters should not criticize their editors. If the who … Stylebook phrase were deleted, this meaning would not be changed.)
USE OF WHO, WHOM, THAT, WHICH. See separate entries on that (conjunction); that, which (pronouns); who, whom.
That is the preferred pronoun to introduce essential clauses that refer to an inanimate object or an animal without a name. Which is the only acceptable pronoun to introduce a nonessential clause that refers to an inanimate object or an animal without a name.
The pronoun which occasionally may be substituted for that in the introduction of an essential clause that refers to an inanimate object or an animal without a name. In general, this use of which should appear only when that is used as a conjunction to introduce another clause in the same sentence: He said Monday that the part of the army which suffered severe casualties needs reinforcement.
See that (conjunction) for guidelines on the use of that as a conjunction.