Discontinuity (linguistics)

In linguistics, a discontinuity occurs when a given word or phrase is separated from another word or phrase that it modifies in such a manner that a direct connection cannot be established between the two without incurring crossing lines in the tree structure. The terminology that is employed to denote discontinuities varies depending on the theory of syntax at hand. The terms discontinuous constituent, displacement, long distance dependency, unbounded dependency, and projectivity violation are largely synonymous with the term discontinuity. There are various types of discontinuities, the most prominent and widely studied of these being topicalization, wh-fronting, scrambling, and extraposition.

Discontinuities should be distinguished from inversion and shifting, two mechanisms that result in non-canonical word order but that do not necessarily incur discontinuities depending on the theory of sentence structure one assumes (e.g. dependency- or constituency-based). Natural languages vary with respect to the types of discontinuities that they permit. The fixed word order of English allows for relatively few discontinuities compared to, for instance, the Slavic languages, which are much more permissive. Even compared to a closely related language such as German, English is rigid, allowing few discontinuities.

Projectivity is a principle of tree structures by which discontinuities are identified and defined. A tree structure is said to be projective if there are no crossing dependency edges and/or projection lines. If there are crossing edges/lines however, then the structure is non-projective and contains one or more projectivity violations. The concept and terminology of projectivity is associated most with the structures of dependency grammar, although the concept is just as applicable to the structures of phrase structure grammars (= constituency grammars). The discussion here considers projectivity first in terms of dependency-based structures and then in terms of constituency-based structures.

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