Nonrestrictive clauses and phrases - Spelling - Part III. Style 20 spelling

A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Nonrestrictive clauses and phrases
Spelling
Part III. Style 20 spelling

Use paired commas to set off a nonrestrictive clause. A clause is nonrestrictive if it is not necessary to uniquely identify the noun it modifies.

These five books, which are on reserve in the library, are required reading.

Here, the noun phrase These five books uniquely identifies the books that the writer has in mind; the nonrestrictive clause is not necessary to identify the books further. On the other hand, in the following sentence, the dependent clause (that are required reading) is restrictive, because it identifies the specific books that are required reading. Commas are therefore not used around the clause.

The books that are required reading are on reserve in the library.

Although which is often used with restrictive clauses, careful writers preserve the distinction between restrictive that (no comma) and nonrestrictive which (comma).

The same principles apply to restrictive and nonrestrictive phrases.

The president, wearing a red dress, attended the conference.

The woman wearing a red dress is the president.