Defining & Non Defining Relative Clauses

RELATIVE CLAUSES

1. Defining relative clauses

If you look up the words  “conductor”  or “doctor” or “liar” in the Oxford Advanced Learner's Dictionary, you will find the following explanations (the words in parenthesis are not given in the dictionary, nor are italics used):

(A) conductor (is a) person who collects fares on a bus or tram. 

(A) doctor (is a) person   who has been trained in medical science.

(A) liar (is a) person who habitually tells lies.

If we omit the words in italics, we learn only that a conductor is a person, a doctor is a person, and a liar is a person, and we would clearly regard such explanations as unsatisfactory. The “persons” are defined, or distinguished from each other (and from any others one could think of), by the relative clauses in italics: the relative clauses are defining.

The definition of conductor is no longer simply “a person”, but “a person who collects fares on a bus or tram”. The relative clause is an essential part of the whole definition, and cannot be omitted if the sentence as a whole is to make useful sense. Similarly, it would be impossible to answer the following question without the defining relative clause in italics:

What do we call a person who habitually tells lies?

The answer is, of course:

A person who habitually tells lies is called a liar.

Again, the answer would be incomplete without the defining relative clause in italics. The subject of the sentence is no longer simply 'a person', but 'a person who habitually tells lies’.

All these examples show that the defining relative clauses provide an indispensable definition of the word `person' (called the antecedent, that is, the word to which the relative clause relates). They are not separated from the antecedent by commas in writing, nor by a pause in speech. 'This is a basic feature of all defining relative clauses.

2. Non-defining relative clauses

The Victoria Line, which was opened in March 1969, was London's first complete  new tube for 60 years.

[1] If we omit the relative clause (in italics), we are left with the statement “The Victoria Line was London's first complete new tube for 60 years”. The relative clause gives additional information about the antecedent (Victoria Line), but does not define it: the “Line” in question is already sufficiently defined by “Victoria”.

The relative clause is in this case called non-defining (or parenthetical), and is enclosed by commas. Whether we include the clause or not, the meaning of the main clause remains exactly the same.

In fact, the main clause and relative clause could (though with less economy) be presented as two separate statements:

The Victoria Line was London's first complete new tube for 60 years. It was opened in March 1969.

The relative clause could even be represented by an independent clause in parenthesis. In this case, it is clearly seen as incidental information, mentioned “by the way”:

The Victoria Line (it was opened in March 1969) was London's first complete new tube for 60 years.

[2] The omission or insertion of commas may represent a difference in meaning between two otherwise identical sentences:

a  They have two children who are still at school. (Defining)

b  They have two children, who are still at school. (Non-defining)

The absence of a comma after children in a implies that they have other children besides the two at school:

They have two children who are still at school and (e.g.) one who goes out to work.

Sentence b, on the other hand, implies that they have only two children, both of school age.

[3] A further point of contrast between defining and non-defining clauses is that the relative pronoun cannot be omitted in non-defining clauses, even if it is not the subject of its clause:

    a The Victoria Line, which was opened in 1969, was London's first complete new tube for 60 years. (which = subject) 

   b The Vittoria Line, which the Queen opened in 1969, was London's first complete new tube for 60 years. (which = object)

You will find many examples in modem written English where commas are omitted before non-defining relative clauses. The writer's meaning may be perfectly unambiguous without the use of commas, or there may be stylistic reasons for omitting them. Nevertheless, students are advised to follow the 'rules' of punctuation illustrated in the above examples, since observation of these patterns is less likely to lead to confusion.