It’s one of the most commonly-debated arguments in the English language, having toppled monarchies, spurred divorces, and caused more than one broken bone here at Wordsworks Towers (we’re nothing if not passionate about our business copywriting).

So, it’s finally time to put the debate to rest: ‘which’ or ‘that’: which is the correct one to use in different contexts?

Let’s start with an example.

‘Food that is undercooked usually makes people unwell.’ We all know that’s the case – but culinary health and safety aside, should the correct sentence actually read, ‘Food which is undercooked usually makes people unwell’?

This is where Jennifer Aniston swooshes her hair around and tells you to ‘get ready for the science part’. The decision rests on the type of noun you’re modifying, and whether it’s a ‘restrictive’ or ‘non-restrictive’ clause. For a restrictive clause you should use ‘that’, and for a non-restrictive clause you should use ‘which’.

So, how to distinguish your restrictive from your non-restrictives?

Ask yourself whether the sentence in question would be fundamentally incorrect if you were to remove the central clause entirely. In the above example, if we lose the clause ‘that is undercooked’, we’re left with ‘Food usually makes people unwell.’ That is plainly untrue. Therefore, we’re almost certainly dealing with a restrictive clause.

A restrictive clause is so-called because (as its name would suggest) it ‘restricts’ the noun to which it refers. In the above example, ‘Food that is undercooked’ refers specifically to ‘undercooked food’ – not all food. Therefore, ‘that is’ is the right option.

So, when might we use the alternative choice – ‘which is’? Logically, it’s for non-restrictive clauses – namely, in cases where the removal of the clause wouldn’t crucially change the meaning of the sentence.

Here’s an example. “Sunburn, which is painful, can often be alleviated with calamine lotion.”

Here we have the non-restrictive clause ‘which is painful’. Remove that clause from the sentence and you’re left with ‘Sunburn can often be alleviated with calamine lotion.’ Still true.

If you’re still unsure, ask yourself whether you’re inclined to include commas around the clause, as in the sunburn example above. More often than not, that will indicate a non-restrictive clause.

Now you can proceed happily with your lives safe in the knowledge there’s one less social faux pas you’re liable to make in polite company…