“Which” and “That”: The Comma Tells You Which One

Two sentences that differ by a word and a comma, and mean different things:

• The books that were damaged have been replaced. Only the damaged ones were replaced.
• The books, which were damaged, have been replaced. All the books were damaged, and all were replaced.

Same words, different facts.

The two kinds of clause

Restrictive clauses narrow down which thing you mean. They are essential — remove one and the sentence changes meaning. They take that and no commas.

Non-restrictive clauses add extra information about something already identified. They are removable. They take which and sit between commas.

The removal test

Take the clause out and see whether the sentence still says what you meant.

• The books have been replaced. If you meant only the damaged ones, this is now wrong — so the clause is essential: that, no commas.
• The books have been replaced. If all of them were replaced, this is still true — so the clause is extra: which, commas.

A second test: could you put the clause in brackets? If yes, it is non-restrictive.

The transatlantic difference

This is where the advice diverges, and it is worth knowing which rule you are being held to.

American usage, following Chicago and most style guides, enforces the split: that for restrictive, which for non-restrictive.

British usage permits which in restrictive clauses. The books which were damaged have been replaced is unremarkable in British writing.

The distinction was largely popularised by Fowler in 1926 as a proposal for clarity rather than a description of usage. American editors adopted it; British ones mostly did not.

The comma is the part that is not optional in either country. A non-restrictive clause takes commas regardless of which pronoun you use, and that is where genuine ambiguity comes from.

Who, whose, where

The same restrictive and non-restrictive logic applies to all relative pronouns:

• The candidate who applied last was hired. Identifies which candidate.
• Maria Hall, who applied last, was hired. She is already identified.

For people, use who. That is acceptable for people and reads as slightly cold to some readers: the man that called against the man who called.

Where it matters most

In contracts and technical documentation, this is not a stylistic question.

• Employees who work overtime will be paid extra. Only those employees.
• Employees, who work overtime, will be paid extra. All employees work overtime, and all get paid extra.

Two commas, an entirely different obligation. This is the same class of problem as the missing serial comma in the Maine overtime case, and it is why legal drafting takes punctuation seriously.

Checked against Merriam-Webster, the Chicago Manual of Style and the AP Stylebook. Spotted an error? Tell us — see our editorial policy.