That vs Which

Without the witchcraft.

The rule (short, sharp)

• That: for essential information.

  • (Restrictive clauses which are necessary to identify the noun.)

• Which: for non-essential information that are set off by commas.

  • (Non-restrictive clauses adding extra information.)

(In British English, which is sometimes used for restrictive clauses too, but many style guides favour that to avoid ambiguity.)

Test (use it in three seconds)

Remove the clause.

• If the core meaning collapses → restrictive → no commas → that.

• If the core meaning survives → non-restrictive → commas → which.

Before / After (see it work)

Restrictive (essential):

• The book that won the prize is sold out.
  (Which book? The prize-winning one. No commas.)

Non-restrictive (extra):

• The book, which won the prize, is sold out.
  (Core idea: the book is sold out. Prize detail = extra. Commas.)

More pairs:

• We need the data that covers Q4.

• The Q4 data, which arrived late, is now uploaded.

• She wore the shoes that matched the dress.

• Her shoes, which cost a fortune, have finally paid off.

Why does it matter?

Because meaning hangs on it. A restrictive clause (usually with that) tells us which one you mean; a non-restrictive clause (with which, comma-hugged) adds extra information you could remove without changing the core point.

Use the wrong one, and you risk ambiguity, waffle, or—worse—misinterpretation.

• Clarity:

  • The contract that expires in June needs revising.
    (Only the June-expiring contract.)

  • The contract, which expires in June, needs revising.
    (The contract—there’s only one under discussion—needs revising; its June expiry is extra information.)

• Accuracy & Liability: In legal, technical, or academic writing, the wrong choice can broaden or narrow a definition unintentionally. That may cost you more than a comma.

People (brief detour)

Use who for people:

• The editor who cut the paragraph was right.

Non-restrictive:

• The editor, who has impeccable taste, was right.

Micro-practice (do it now)

Open a previous email, presentation, message and find one which/that.

• If the clause is essential, change to that and remove commas.

• If it’s extra, change to which and add commas.

Yours, briefly and precisely,
Thomas

P.S. Think of that as security, which as jewellery: if it’s essential, wear that; if it just sparkles, wear which—with commas, naturally.