A subordinate clause is part of a sentence that depends on a main clause for its meaning. Relative clauses, which you may encounter in both defining and non-defining form, are types of subordinate clauses that work in specific ways.
You can normally recognise a relative clause within a sentence because it will begin with a word such as ‘which’, ‘that’, ‘who’, ‘whose or ‘whom’.
First, we will focus on the uses of ‘that’ and ‘which’ in defining and non-defining clauses. The word ‘define’ gives you a clue to the purpose of the clause.
A defining (relative) clause does just that: it defines the noun, or subject, of the sentence it is part of. The defining clause is an essential part of the sentence and cannot be removed without changing the sentence’s meaning (although the sentence may still make sense, it will not make the same sense). A defining clause removes ambiguity and creates clarity.
Have a look at the following sentence:
The witch’s broomstick that was made of willow branches was her favourite.
Here, the defining clause begins with ‘that’, and defines the witch’s favourite broomstick as the one made of willow. Without the defining clause ‘that was made of willow branches’ we would not know exactly which broomstick was her favourite. Any ambiguity has been removed and we know exactly which broomstick she prefers.
Here’s another sentence:
The witch’s broomstick, which she had won in a cauldron-stirring contest, flew especially well over water because it was made from willow.
In this sentence, the relative clause is non-defining and begins with ‘which’. This relative non-defining clause gives us extra information about the broomstick, but is not essential to the overall meaning of the sentence. Notice that the non-defining relative clause is also separated from the main clause by commas. Here is the same sentence without the relative non-defining clause:
The witch’s broomstick flew especially well over water because it was made from willow.
Sometimes, depending on the noun, or subject of the sentence, the words ‘who’, ‘whom’ or ‘whose’ can be used instead of ‘which’:
The witch, who wore a silver-lined cloak, was a new member of the Full Moon Committee.
The word ‘who’ can be used because a witch is a person; if a thing were being discussed, then the preferred word would be ‘that’:
The silver-lined cloak that belonged to the new witch was much admired.
Of course, as with most grammar rules, you will find conflicting advice and examples. Regarding the last two examples, some grammar guides consider it acceptable to use ‘who’ and ‘that’ interchangeably, whether the sentence’s subject (noun) is a person or a thing. In addition, some grammar guides allow the use of ‘which’ in a defining clause (The witch’s broomstick that was made of willow branches was her favourite). However, when you are writing and reviewing your own text it is often best to use the most generally accepted rule, and the one that gives your writing the greatest clarity, simplicity and elegance.
Updated 04 October 2018