Supplementation (from CGEL, by Rodney Huddleston & Geoffrey K. Pullum)

Supplementation
We turn now to supplementation constructions, illustrated in such examples as:

[1]
i   Pat — the life and soul of the party — had invited all the neighbours.
ii  The best solution, it seems to me, would be to readvertise the position.
iii Jill sold her interest shares in January — a very astute move.

The underlined expressions are supplements, elements which occupy a position in linear sequence without being integrated into the syntactic structure of the sentence.

General properties of supplementation
In the clear and central cases, supplements have the character of interpolations or appendages. An interpolation, as in [1i–ii], is located at a position between the beginning and end of a main clause: it represents an interruption to the flow of the clause. An appendage is attached loosely at the beginning or end of a clause. In speech, supplements are marked as such by the prosody: they are intonationally separate from the rest of the sentence. In writing, they are normally set off from the rest of the sentence by punctuation marks — commas, or stronger marks such as dashes, parentheses, or (in the case of appendages in end position) a colon. Punctuation allows for different degrees of separation, as described in Ch. 20 §§4–5.

❑ Supplementation in relation to dependency constructions and coordination
It is the lack of integration into the syntactic structure that distinguishes supplements from dependency constructions and coordination. But supplementation is like coordination in being non-headed: since the supplement is not integrated into the structure it cannot function as a dependent to any head. The three types of construction are thus distinguished as shown in:

[2]                                                            _{INTEGRATED?              HEADED?}
i   DEPENDENCY CONSTRUCTION                    Yes                 Yes
ii  COORDINATION                                         Yes                  No
iii SUPPLEMENTATION                                    No              .   No

It should be noted, however, that expressions introduced by a coordinator can have the status of supplements rather than coordinates in an (integrated) coordination construction:

[3]
Jill — and I don't blame her — left before the meeting had ended.    [supplement]

In spite of the and, the underlined clause is an interpolation, and is clearly not of equal syntactic status with the clause Jill left before the meeting had ended. We thus treat [3] as an instance of supplementation, not coordination, such as we have in Jill left before the end of the meeting and I was sorely tempted to follow her.

❑ Supplements and anchors
Although supplements are not syntactically dependent on a head, they are semantically related to what we will call their anchor⁶⁹. In [1i] the anchor is the NP Pat, while in [1ii]iii] and [3] it is a clause — the clause which the supplement interrupts or follows. Other possibilities are shown in [4], where bold marks the anchor, underlining the supplement:

[4]
i   When the patient closed his eyes, he had absolutely no spatial (that is, third-dimensional) awareness whatsoever.
ii  The goal is to produce individuals who not only possess ‘two skills in one skull’, that is, are bicultural, but can also act as human links between their two cultures.

In [1] the anchor is the adjective spatial (which functions as attributive modifier to the noun awareness); in [ii] it is the VP possess ‘two skills in one skull’ (the first coordinate in a VP-coordination).
A supplement must be semantically compatible with its anchor. Compare, for example:

[5]
1   This stipulation — that the amount of damages not be divulged — was ignored.
ii  ^#This stipulation — whether the press could be informed — was ignored.

The supplement in [1] is a declarative clause and as such can appropriately combine with the anchor this stipulation. The anomaly of [ii] stems from the fact that the supplement is an interrogative clause and hence is not semantically compatible with its anchor.

⁶⁹Some writers use the term ‘host’, but we have avoided this because we use it elsewhere in its primary sense, where it applies to the word to which a clitic is attached (see Ch. 18, §6.2).

❑ Supplements vs dependents
Semantic compatibility vs syntactic licensing of complements
The restriction illustrated in [5] is comparable to that which holds between a complement and the head nominal in NP structure:

[6]
1   This stipulation that the amount of damages not be divulged was ignored.
ii  *This stipulation whether the press could be informed was ignored.

This time, underlining indicates the complement, bold the head. The noun stipulation licenses a declarative content clause as complement, but not an interrogative, so [ii] is inadmissible.
There is a significant difference between [5] and [6], however. The integrated construction shown in [6] requires that the complement by syntactically licensed, whereas in supplementation it is, as we have said above, a matter of semantic compatibility. Compare:

[7]
i   a. The stipulation that Harry could not touch the money until he was eighteen annoyed him enormously.
ccb. *The codicil that Harry could not touch the money until he was eighteen annoyed him enormously.
ii  a. This stipulation — that Harry could not touch the money until he was eighteen — annoyed him enormously.
ccb. The codicil in the will — that Harry could not touch the money until he was eighteen — annoyed him enormously.

The examples in [1] belong to the integrated head + complement construction. Stipulation licenses a declarative complement, but codicil does not: hence the ungrammaticality of [ib]. In [ii] the content clause is a supplement, interpreted as specifying the content of its anchor NP. And this time the codicil example is acceptable: the NP it heads denotes an addition to a will and hence has propositional content which can be specified by a declarative content clause.
As a second illustration of the difference between the integrated and non-integrated constructions, consider:

[8]
i   a. The question (of) where the funding would come from wasn't discussed.
ccb. *The thing (of) where the funding would come from was rather more important.
ii  a. The second question — where the funding would come from — wasn't discussed.
ccb. The thing they didn't discuss — where the funding would come from — was rather more important.

Here the content clause is interrogative. In [ia] it is a dependent within the NP headed by the noun by the head noun: , so [ib] is ungrammatical. In [ii] the interrogative clause is a supplement and is subject to the weaker constraint that it be semantically compatible with its anchor. Example [iib] is therefore admissible because the anchor NP as a whole denotes a potential topic of discussion, so that the content of this topic can be specified by means of an interrogative clause supplement.

Form and interpretation of supplements realised by clauses
A further important difference between supplements and dependents is that the former may be realised by main clauses with their own illocutionary force:

[9]
Sue felt — can you blame her? — that she was being exploited.
The supplement here has the form and interpretation of a main clause: there is no change in form or loss of independent illocutionary force such as is found with clauses realising a dependent function.

❑ Supplements and non-restrictiveness
By virtue of not being integrated into the syntactic structure, supplements are necessarily semantically non-restrictive. Compare, for example, [8ia–iia]. In the former, the integrated construction, the content clause is semantically restrictive, distinguishing the question being referred to from other questions. It provides the identifying information that makes it appropriate to use the definite article the. By contrast, in [iia], the supplementation construction, the second question by itself constitutes a definite referring NP. The supplement doesn't serve to distinguish one second question from other second questions: it doesn't restrict the denotation of the head nominal.
The same contrast between dependency and supplementation constructions is commonly found with relative clauses and appositives:

[10]
i   a. The necklace which her mother gave to her was in the safe.             i i  [modifier]
ccb. The necklace, which her mother gave to her, was in the safe.               [supplement]
ii  a. They are working on a new production of the opera ‘Carmen’.                [modifier]
ccb. Bizet's most popular opera, ‘Carmen’, was first produced in 1875.     [supplement]

In [ia] the relative clause is a modifier of the head noun necklace and serves semantically to identify which necklace is being referred to, but in [ib] it is a supplement to the anchor NP the necklace, which is assumed to be identifiable independently of the information given in the relative clause. Similarly, in [iia] the appositive Carmen is a modifier of opera, identifying which opera is being referred to, while in [iib] it is a supplement to the anchor NP Bizet's most popular opera, and since there can be only one entity satisfying that description the supplement is again non-restrictive.
However, we have noted in our description of relative clauses and appositives that the integrated construction is not necessarily semantically restrictive — see Ch. 12, §4.2, and Ch. 5, §14.3, respectively. Compare, then:

[11]
i   The father who had planned my life to the point of my unsought arrival in Brighton took it for granted that in the last three weeks of his legal guardianship I would still act as he directed.
ii  This is my husband George.

In [1] the relative clause doesn't distinguish one father from another: the narrator has only one father, so the modifier provides non-restrictive information about him. And [ii] does not convey that the speaker has more than one husband.
It is for this reason that we have departed from the traditional account of relative clauses, in which the two main constructions are distinguished as ‘restrictive’ and ‘non-restrictive’. A distinction in terms of integrated versus supplementary reflects the semantic difference more accurately and also matches the prosodic difference that distinguishes them in speech. It enables us, moreover, to capture the similarity between the unintegrated relatives and other elements that are semantically, prosodically, and syntactically unintegrated with the rest of the sentence: these can all be subsumed under the concept of supplement.

❑ Syntactic representation of supplementation
A supplement, as we have seen, requires a semantically appropriate anchor: it cannot occur, as a supplement, without the anchor. Thus if we drop the anchor from [10ib], for example, the result is ungrammatical: *Which her mother gave to her, was in the safe. And if we drop it from [10iib] ‘Carmen’ takes on the status of an integrated dependent, the subject: ‘Carmen’ was first produced in 1875. For this reason, we take the anchor and its supplement to form a construction: a supplementation construction. But the lack of integration of the supplement into the syntactic structure means that there is no good reason to treat the supplementation as a syntactic constituent. We propose, therefore, that in the syntactic representation supplements should be kept separate from the tree structure, related to their anchors by some different notational device, as in [12]:

[12]

In [12i], representing the structure of example [1iii], the supplement a very astute move has the clause Jill sold her internet shares in January as its anchor: this is shown by the broken line leading from the functional label ‘Supplement’ to the category label ‘Clause’. Similarly in [12ii] the broken line shows that the relative clause is a supplement to the NP the necklace.