Apposition is a syntactic relation between two constituents H and A as follows:

1. The constituents are nominals or NP; but at least one of them is an NP.
2. H is the head, A is the appositive.
3. H and A together form an NP.
4. H and A are in a sociative relation: although H is the head, A does not depend on H.
5. H and A share their referent.

and illustrate two subtypes of apposition, to be taken up presently.

One of the two components can be a nominalization. In , it is an adnominal substantive clause; in , it is a non-restrictive relative clause.

In , the first component is a nominalization; the following head is an NP.

In several languages, the class of appositive NPs may be subdivided by a prosodic criterion:

• In close apposition, the complex NP is one prosodic unit.
• In loose apposition, the complex NP consists of two prosodic units, with an intonation break and a possible pause between them.

By this criterion, , and show close apposition, while and show loose apposition.

Semantically, an appositive may restrict the head notion, as it does in , and ; or it may add nothing to the identification of the referent, as is the case in the other above examples.

As is obvious, there is some correlation – in fact, a perfect match in the examples given – between the prosodic and the semantic subdivision. To the extent that close apposition is semantically restrictive, it is like attribution. For instance, in , the substantive clause can be substituted by a restrictive relative clause (cf. the section on attribution), with no other change in formal structure and with the same semantic effect of restricting the head concept by the subordinate clause.

A correlation of the prosodic difference with a syntactic difference would be desirable. In the simplest case, the head in loose apposition would be an NP, while it would be a nominal in close apposition. Then the situation of restrictive vs. non-restrictive relative clauses would just be a special case of the contrast between attribution plus close apposition, on the one hand, and loose apposition, on the other. Unfortunately, this does not appear to work out. The head in is an NP, but it is restricted by the prenominal appositive. It appears that this is one of the many cases where one has to abide by a less than perfect match between function and structure.

Finally, the definition given above assumes that the appositive construction has a head. The head of a syntagma is that component which determines the distribution of the whole. However, this is hard to operationalize in a construction which consists of two nominal constituents:

• In close apposition, if one of the members is an NP, one might conclude that this is the head. In , the proper name is an NP, while teatro is not. Unless the first constituent is el teatro, Colón would have to be the head. However, since the postnominal NP restrictively specifies the preceding common noun, one can also assume that the latter is the head. As a consequence, there is some dispute in the literature as to which is the head in constructions like .
• In loose apposition, it appears that the criterion to determine the head is quite another one: simply the first constituent is the head.