Misc.: Essential/nonessential information

January 26, 2006: Issue 124

Not sure whether to set off a phrase or clause with commas? You might need a quick refresher in essential and nonessential information, also called restrictive and nonrestrictive.

If the information is essential, or restrictive, it is essential to understanding exactly what person or thing you’re describing; it restricts the number of people or things fitting that description. Whether a word, a phrase, or a clause, it doesn’t need commas:

Her co-worker Julia served as a sounding board.
A door that was salvaged from the Thorkelsons’ old home adds old charm to the entrance.

(“Her co-worker” and “a door” define broad categories. “Julia” and “that was salvaged from the Thorkelsons’ old home” narrow the fields.)

Nonessential, or nonrestrictive, information provides additional detail but doesn’t identify the person or thing more specifically. Set off these words, phrases, and clauses with commas.

Her oldest sister, Julia, served as a sounding board.
The Thorkelsons’ front door, which was salvaged from their previous home, adds old charm to the entrance.

(She has only one “oldest sister”; the Thorkelsons have only one “front door.” “Julia” and¬† “which was salvaged from their previous home” don’t narrow the descriptions any further.)

That which confounds us

The essential/nonessential question also determines whether you use “that” or “which.” Essential information uses “that.” Nonessential information uses “which.”

One of the houses that I saw yesterday needs some work.
Ali’s house, which I saw yesterday, needs some work.

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