That vs. Which

That and which are common everyday words that most people use without a second thought, which often means they use them incorrectly. Understanding their differences and when to use each one will noticeably improve your writing.

Both that and which are types of relative pronouns. This means they are used to introduce more information to the sentence by means of a relative clause. Simply, that and which are used to introduce a piece of information, joining two clauses and providing some required information. Although both are similar in meaning, they cannot be used interchangeably.

When using a restrictive clause, that is the correct word choice. That is used when referring to people or things, and so often limits or restricts the information within a sentence. That will identify a specific person or object to give clarity, for example:

He picked up the bird that fell from the tree.

That helps to identify which bird is being talked about, but does not add any further information.

Which is used within a non-restrictive clause. This means, which is providing more information on the subject; not restricting it. When using which, the piece of information being added will be set with commas around it as it is non-essential and could easily be left out without affecting the sentence meaning. For example:

The bird he picked up, which fell from the tree, was brown. 

In the example above, the bold clause could be removed and the sentence would still make sense.

To note, both that and which can be used in more than just the ways set out here, and so will not always follow these rules. Which is often used to question, and that, to respond. In these instances, different rules apply. 

To conclude, discussing clauses can seem quite complex, but the use of that and which can become second nature with practice. Just remember, that is used to identify and which is used to supply further information. Ask yourself: can this clause be removed without changing the meaning of the sentence? If it cannot and the information is required, then you need to use that. If the information can be removed and the sentence is still understood, then which is the right choice.