Defining Vs Non-Defining Relative Clauses

Consider the following sentence:

The house that Jack built, which is a British nursery rhyme, is a 'cumulative' tale.

There are two relative clauses:

1. that Jack built
2. which is a British nursery rhyme

The first of these is a defining relative clause, while the second is a non-defining relative clause.

On the SFL model, the defining relative clause is embedded as Qualifier in a nominal group, while a non-defining relative clause is a ranking clause within a clause complex.  In the example above, the non-defining relative interrupts the clause on which it is dependent, and is termed 'enclosed'.

The distinction between the clauses is indicated by the following notation:

The house [[that Jack built]] <<which is a British nursery rhyme>> is a 'cumulative' tale.

Let's take a trinocular perspective on these types of relative clauses.

The view from above (the meaning being realised by the wording):

In terms of the logical semantic relations between the relative clause and the item it modifies, both types involve hypotactic elaboration.

The relation is hypotactic because each relative clause is dependent on the item that it modifies:

• [[that Jack built]] is dependent on the house
• <<which is a British nursery rhyme>> is dependent on the house [[that Jack built]]

The relation is elaborating because each relative clause expands the item it modifies by further specifying or describing it:

• [[that Jack built]] further specifies/describes the house
• <<which is a British nursery rhyme>> further specifies/describes the house [[that Jack built]]

Hypotactic elaboration is what the two types of relative clause have in common.  But they also significantly differ in meaning.  A defining relative clause specifies a particular subset of a general class, whereas a non-defining relative clause 'adds a further characterisation of something that is taken to be already fully specific' (Halliday & Matthiessen 2004: 400).

The view from below (how the wording is realised):

In terms of intonational realisation, the two types of relative clause differ markedly.

In the unmarked case — unless there are textual pressures for doing otherwise — the defining relative clause is realised within the same tone group realising the nominal group in which it functions as embedded Qualifier.  That is, both are packaged as a single unit of information.  So, in the above example:

• the house and [[that Jack built]] are realised within the same tone group.

On the other hand, in the unmarked case, non-defining relative clauses are realised by their own separate tone group.  That is, the relative clause and the item that it modifies are each packaged as  distinct units of information.  So, in the above example:

• the house [[that Jack built]] and <<which is a British nursery rhyme>> are each realised as distinct tone groups, thereby construing them as distinct units of information.

The view from roundabout (systemic relations at the level of wording):

In terms of systemic possibilities, the two type of relational clause differ markedly. This is a consequence of defining relative clauses being embedded as Qualifiers in nominal groups, and non-defining being ranking clauses in their own right.

Dependent elaborating clauses, of which non-defining relative clauses are a type, can be given thematic prominence in a clause nexus.  A non-finite agnate (see Halliday & Matthiessen 2004: 404) of the example above is the following:

• A British nursery rhyme, the house that Jack built is a 'cumulative' tale.
  

(If you don't understand how this clause is agnate with the original, see http://thoughts-that-cross-my-mind.blogspot.com.au/2013/09/michl-odonnell-on-non-defining-relative.html)

However, as embedded clauses, defining relative clauses are not given thematic prominence in a clause.  For example, we do not say:

• Jack built, the house, which is a British nursery rhyme, is a 'cumulative' tale.

Moreover, the lexicogrammatical domain which is elaborated by a non-defining relative clause may be a whole clause or any of its constituents, as Halliday & Matthiessen (2004: 400) point out, whereas the domain elaborated by a defining relative clause is restricted to a nominal group.

In summary, taking a trinocular vision demonstrates that defining and non-defining relative clauses share common logical semantic features, but differ markedly in every other respect.

As Halliday (2008: 6) has pointed out:

The boundaries of any grammatical category are likely to be fuzzy […] — such indeterminacy is a general property of the grammar.  The grammarian attempts to define each category as accurately as possible, looking at it from three different angles: its systemic environment (contrast with other term or terms in the system, and the relationship of that system to other systems); its meaning (proportionality in semantic terms), and it form.  In other words, the grammarian adopts a “trinocular” perspective on the stratal hierarchy so that every category is viewed “from round about”, “from above” and “from below”.  And since the views from these different angles often conflict, assigning instances to a particular category involves some degree of compromise, where criteria will depend on the purposes of the description.