Part xi. When to Use Which & That

                                    

                                          
This part was requested by zuko_42.
                          
To understand when to use that or which, it's important to first understand clauses. I mentioned this briefly in the first part of this section. There are many different types of clauses, but the one's we're going to focus on are relative clauses.
                          
Relative clauses can contain a subject, verb, and a relative pronoun, though not all are needed.
                          
The relative pronouns are: who, whose, when, where, which, and that.
                          
Relative clauses are sometimes called adjective or adjectival clauses because they describe nouns like adjectives do.
                          
There are two types of relative clauses—defining and non-defining.
                          
Defining Clauses
                          
A defining clause (also called an essential clause or a restrictive clause) gives information essential to the meaning of the sentence.
                          
Defining clauses give essential information about the main noun. Often, they answer the question: "Which one?"
                          
That is used in defining clauses.
                          
Here's an example:
                          
My bike that has a broken seat is in the garage.
                          
In this sentence, you understand that the speaker has at least one other bike. Specifically, the bike he's talking about is distinguished from his other bikes by its broken seat.
                          
If you removed the clause "that has a broken seat," you would lose the implication that he owns more than one bicycle, and even if you somehow knew about the other bikes, you wouldn't know which one was in the garage.
                          
Non-Defining Clauses
                          
If you haven't already guessed, non-defining clauses do not define nouns. But what do they do?
                          
Unlike defining clauses, non-defining clauses (also called nonessential or nonrestrictive clauses) don't limit the meaning of the sentence.
                          
You might lose interesting details if you remove them, but the meaning of the sentence wouldn't change. Sometimes, these phrases are set off by commas.
                          
Non-defining relative clauses provide supplementary information. However, the information is not key to the meaning of the sentence. In fact, the sentence would still make sense if you removed the non-defining clause.
                          
Which introduces non-defining clauses.
                          
My bike, which has a broken seat, is in the garage.
                          
Here, the broken seat is simply a description of the bike in the garage. There's no implication that the speaker owns more than one bike.
                          
Do you see the difference? Perhaps a little mnemonic device will help you to remember how to choose between that or which.
                          
How to Remember the Difference Between That and Which
                          
Because non-defining clauses add removable information, it's easy to remember to use which if you think of a plastic sandwich bag. They are disposable, and so are clauses with which.
                          
Look at these example sentences with and without non-defining clauses.
                          
The country of Costa Rica has extensive measures in place to protect endangered rainforest animals. 
                          
This sentence does not have a relative clause.
                          
The country of Costa Rica, where extinct species such as the golden toad once thrived, has extensive measures in place to protect endangered rainforest animals.
                          
The non-defining clause "where extinct species such as the golden toad once thrived" tells us more about Costa Rica, but nothing that is essential to understanding the sentence.
                          
Commas Surround Non-Defining Clauses
                          
Besides the type of information added (essential vs. nonessential), the use of commas is different between the two types of clauses.
                          
Commas set off non-defining clauses from the rest of the sentence.
                          
Defining clauses do not need commas.
                          
For example:
                          
Classes that are held on Wednesdays are in building 206.
                          
Leap years, which have 366 days, contain an extra day in February.
                          
In the first sentence, the words "that are held on Wednesdays" are limiting the type of classes that we're talking about. (We're not talking about all the classes, only the ones held on Wednesdays.) Therefore, we use that.
                          
In the second sentence, the words "which have 366 days" are adding information. Therefore, we use which surrounded by commas.
                          
In This Difference Worth Bothering With?
                          
Let's face it, most people are unaware of the guideline set out above. Thus, we can confidently say that most people probably use that and which interchangeably. In most instances, this doesn't cause undue confusion.
                          
In formal business or technical communications (for example, contracts, tenders or technical specifications), though, such ambiguities can give rise to serious legal and financial problems.
                          
Solidify Your Understanding
                          
Consider the following sentences. Both are acceptable, but their meanings are subtly different:
                          
The books, which have red covers, are new.
                          
The books that have red covers are new.
                          
In the first example, the words "which have red covers" are adding information about the books. That is, they're telling you more about the books than you'd otherwise have known. (They're red, not some other color.) All of the books are new.
                          
In the second example, the words "that have red covers" are limiting which books we're talking about. We're no longer talking about all the books; we're only talking about the ones with red covers. So this time, only the red books are new.
                          
Let's try another:
                          
Our office, which has two lunchrooms, is located in Cincinnati.
                          
Our office that has two lunchrooms is located in Cincinnati.
                          
These sentences are not the same. The first sentence tells us that you have just one office, and it's located in Cincinnati. The clause "which has two lunchrooms" gives us additional information, but it doesn't change the meaning of the sentence. Remove the clause and the location of our one office would still be clear: "Our office is located in Cincinnati."
                          
The second sentence suggests that we have multiple offices, but the office with two lunchrooms is located in Cincinnati. The phrase "that has two lunchrooms" is known as a restrictive clause because another part of the sentence (our office) depends on it. You can't remove that clause without changing the meaning of the sentence.
                          
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