In Canada, we use a grammar style that takes from both British and American English.
We side with the Americans when it comes to ‘which’ and ‘that,’ two words that have similar but distinctly different functions.
Far too often – and even in many professional documents – ‘which’ and ‘that’ are used interchangeably.
First, we need to understand the difference between restrictive and nonrestrictive clauses.
A restrictive clause is necessary information that differentiates the noun from other possibilities.
A nonrestrictive clause is information that describes the noun as a general truth and can be used as an alternate definition or further description of the noun.
‘That’ introduces a restrictive clause
‘Which’ introduces a nonrestrictive clause
Further, ‘which’ is always set off with commas, while ‘that’ is not.
Check out these examples:
I want to buy a shirt that is blue.
- ‘That’ restricts the noun ‘shirt’ to only ones that are blue. There could be any number of colours and kinds of shirts in the world. We are restricting the shirt I want to one that is blue.
I’ll bring my favourite shirt, which is blue.
- ‘Which’ acts as a further description for the noun ‘favourite shirt’ and is therefore nonrestrictive. It is just a further definition for my favourite shirt. There is only one favourite shirt and it is blue.
See how that works?
I like whales, which swim in the water.
- Swimming in the water is what whales do – all of them do it, so the description is nonrestrictive.
I like whales that are white.
- Not all whales are white, so we restrict the noun to just the white ones.
I saw the movie that I wanted to see.
- ‘That’ restricts ‘movie’ to just the one I wanted to see.
I saw Independence Day, which I wanted to see.
- ‘Which’ works as a definition for Independence Day. It is the movie I wanted to see.
‘Who’ is a word that functions similarly to ‘which’ and ‘that’ and can be used in both restrictive and nonrestrictive clauses.
We use ‘who’ for human beings and ‘that’ or ‘which’ for objects.
I want to see the man who called.
- ‘Who’ restricts ‘man’ to just the one who called.
I want to see my wife, who is beautiful.
- There is only one wife, at least in modern society, so ‘who’ acts as a definition and further description for ‘my wife.’
Notice that the same comma rules are followed for all three terms: in nonrestrictive clauses, set off the clause with commas; in restrictive clauses, do not use a comma.