Which vs. That
The words which and that are versatile words that can be used in many different ways. However, it is when they are used to introduce dependent clauses that people often have trouble choosing which to use. While it may seem nit-picky and you might think that using either one is okay, you can really change the meaning of your sentence depending on the one you choose.
Which and that can both function as relative pronouns, meaning they can introduce relative clauses, which are dependent, adjective clauses giving additional information about a noun in a sentence. Relative clauses can be essential or non-essential. This means that the information they provide about the noun is either necessary to understand the meaning of the sentence or not necessary information.
That is used with essential relative clauses. If the information is necessary for understanding the meaning of the sentence, then you use that.
Which is used with non-essential relative clauses. If the information is not necessary-but just fun, additional information-you use which.
Let's look at some sentences to see how using that or which changes the meaning of the sentence.
1. My dad's car that is red needs to be washed today.
2. My dad's car, which is red, needs to be washed today.
The first sentence uses that and the information is considered essential. This sentence implies that dad has more than one car, and it's the red one that needs to be washed. The second sentence uses which, denoting that the information about the car being red is non-essential. This sentence implies that dad has one car that needs to be washed-and while it's red, that information is not essential to the sentence.
1. The painting that I completed last month won first prize in the county fair.
2. The painting, which I completed last month, won first prize in the county fair.
Again, the first sentence uses that, which means that this painting is one of many that were completed, so we need the essential information that it is the one completed last month that one the prize. The second sentences uses which, so there is only one painting-and it just happened to be completed last month.
So, in summary, depending on which you choose-that or which-to introduce a relative clause, you are sending a very clear message about the meaning of the sentence to your reader.
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