H.W. Fowler (1858–1933). The King’s English, 2nd ed. 1908.
Chapter II. Syntax
a. Defining and non-defining relative clauses.
FOR the purposes of b. and c., all relative clauses are divided into defining and non-defining. The exact sense in which we use these terms is illustrated by the following groups, of which (i) contains defining clauses, (ii) non-defining.
The function of a defining relative clause is to limit the application of the antecedent; where that is already precise, a defining clause is not wanted. The limitation can be effected in more than one way, according to the nature of the antecedent. As a rule, the antecedent gives us a class to select from, the defining clause enables us to make the selection. Thus in our first example the antecedent leaves us to select from the general class of 'men', the defining clause fixes the particular man (presumably the only man, or the only man that would occur in the connexion) 'who called yesterday'. Sometimes, however, the functions of the two are reversed. When we have an antecedent with a superlative, or other word of exclusive or comprehensive meaning, such as 'all', 'only', 'any', we know already how to make our selection, and only wait for the relative clause to tell us from what class to make it. We know that we are to choose 'the best novel': the relative clause limits us to the works of Trollope. We are to choose 'any man' we like, provided (says our relative clause) that he 'knows three words of Greek'. In either case, the work of definition is done by the exclusion (implied in the relative clause) of persons or things that the antecedent by itself might be taken to include.
The point to notice is that, whichever way the defining clause does its work, it is essential to and inseparable from its antecedent. If for any reason we wish to get rid of it, we can only do so by embodying its contents in the antecedent: 'The man in Paris with whom I correspond' must become 'My Paris correspondent'. To remove the clause altogether is to leave the antecedent with either no meaning or a wrong one. Even in such extreme cases as 'the wisest man that ever lived', 'the meanest flower that blows', where the defining clause may seem otiose and therefore detachable, we might claim that future wise men, and past and future flowers, are excluded; but we shall better realize the writer's intention if we admit that these clauses are only a pretence of limitation designed to exclude the reality; it is as if the writers, invited to set limits to their statements, had referred us respectively to Time and Space.
This fact, that the removal of a defining clause destroys the meaning of the antecedent, supplies an infallible test for distinguishing between the defining and the non-defining clause: the latter can always, the former never, be detached without disturbing the truth of the main predication. A non-defining clause gives independent comment, description, explanation, anything but limitation of the antecedent; it can always be rewritten either as a parenthesis or as a separate sentence, and this is true, however essential the clause may be to the point of the main statement. 'Jones', in our last example above, is quoted chiefly as one 'who should know something of the matter'; but this need not prevent us from writing: 'Jones thinks differently; and he should know something of the matter'.
To find, then, whether a clause defines or does not define, remove it, and see whether the statement of which it formed a part is unaltered: if not, the clause defines. This test can be applied without difficulty to all the examples given above. It is true that we sometimes get ambiguous cases: after removing the relative clause, we cannot always say whether the sense has been altered or not. That means, however, not that our test has failed, but that the clause is actually capable of performing either function, and that the main sentence can bear two distinct meanings, between which even context may not enable us to decide. The point is illustrated, in different degrees, by the following examples:
Mr. H. Lewis then brought forward an amendment, which had been put down by Mr. Trevelyan and which provided for an extension of the process of income-tax graduation.—Times.
The former of these is quite ambiguous. The bringing forward of an amendment (no matter what or whose) may be all that the writer meant to tell us of in the first instance; the relative clauses are then non-defining clauses of description. On the other hand, both clauses may quite well be meant to define; and it is even possible that the second is meant to define, and the first not, though the coordination is then of a kind that we shall show under c. to be improper. Similarly, in the second sentence, 'to portend developments' may possibly be complete in itself; the whole might then be paraphrased thus: 'It was thought that the matter would not stop there: but it has'. More probably the clause is meant to define: 'It was held to portend what have since proved to be unrealized developments'. This view is confirmed, as we shall see, both by the use of 'that' (not 'which') and by the absence of a comma before it.
Punctuation is a test that would not always be applicable even if all writers could be assumed to punctuate correctly; but it is often a guide to the writer's intention. For (1) a non-defining clause should always be separated from the antecedent by a stop; (2) a defining clause should never be so separated unless it is either preceded by a parenthesis indicated by stops, or coordinated with a former defining clause or with adjectives belonging to the antecedent; as in the following examples:
The only circumstance, in fact, that could justify such a course...
It will occur to the reader that our last two examples are strictly speaking exceptions to the rule of defining clauses, since they tell us only what is already implied, and could therefore be removed without impairing the sense. That is true to some extent of many parallel defining clauses: they are admissible, however, if, without actually giving any limitation themselves, they make more clear a limitation already given or implied; if, in fact, they are offered as alternative versions or as reminders. Our next example is of a defining clause of the same kind:
This estimate which he gives, is the great groundwork of his plan for the national redemption.—Burke.
The limitation given by 'this' is repeated in another form by the relative clause. 'This estimate, the one he gives, is...'
The reader should bear in mind that the distinction between the two kinds of relative is based entirely on the closeness of their relation to the antecedent. The information given by a defining clause must be taken at once, with the antecedent, or both are useless: that given by a non-defining clause will keep indefinitely, the clause being complete in sense without the antecedent, and the antecedent without the clause. This is the only safe test. To ask, for instance, whether the clause conveys comment, explanation, or the like, is not a sufficient test unless the question is rightly understood; for, although we have said that a non-defining clause conveys comment and the like, as opposed to definition of the antecedent, it does not follow that a defining clause may not (while defining its own antecedent) contribute towards comment; on the contrary, it is often open to a writer to throw his comment into such a form as will include a defining clause. It may even appear from a comparison of the two sentences below that this is the origin of the non-defining clause, (2) being an abbreviation of (1):
In (1), a comment is introduced by 'a man' in apposition with Lewis; 'a man' is antecedent to a defining relative clause; separate them, and the antecedent is meaningless. But next remove the connecting words 'a man', and the relative changes at once its antecedent and its nature: the antecedent is 'Lewis'; the relative is non-defining; and the clause is (not merely contributes to) a comment.
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