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.
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.
The flatter the tree structures are, the fewer projectivity violations they will contain. As structures become more layered, the number of projectivity violations can increase.
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Meaning–text theory (MTT) is a theoretical linguistic framework, first put forward in Moscow by Aleksandr Žolkovskij and Igor Mel’čuk, for the construction of models of natural language. The theory provides a large and elaborate basis for linguistic description and, due to its formal character, lends itself particularly well to computer applications, including machine translation, phraseology, and lexicography. Linguistic models in meaning–text theory operate on the principle that language consists in a mapping from the content or meaning (semantics) of an utterance to its form or text (phonetics).
In syntax, shifting occurs when two or more constituents appearing on the same side of their common head exchange positions in a sense to obtain non-canonical order. The most widely acknowledged type of shifting is heavy NP shift, but shifting involving a heavy NP is just one manifestation of the shifting mechanism. Shifting occurs in most if not all European languages, and it may in fact be possible in all natural languages including sign languages.
Extraposition is a mechanism of syntax that alters word order in such a manner that a relatively "heavy" constituent appears to the right of its canonical position. Extraposing a constituent results in a discontinuity and in this regard, it is unlike shifting, which does not generate a discontinuity. The extraposed constituent is separated from its governor by one or more words that dominate its governor. Two types of extraposition are acknowledged in theoretical syntax: standard cases where extraposition is optional and it-extraposition where extraposition is obligatory.
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