The terminology that is employed to denote discontinuities varies depending on the theory of syntax at hand.
[clarification needed] There are various types of discontinuities, the most prominent and widely studied of these being topicalization, wh-fronting, scrambling,[1] and extraposition.
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.
Dependency grammars have explored the projectivity principle in great detail and have formalized it rigorously.
If crossing lines are obtained in the tree, projectivity has been violated, meaning a discontinuity is present.
[6] The following discussion briefly considers four widely acknowledged types of discontinuities: 1) topicalization, 2) wh-fronting, 3) scrambling, and 4) extraposition.
Wh-fronting (= wh-movement) occurs when a wh-expression appears at the front of the clause in order to focus it.
Scrambling discontinuities are absent from English, but they are very common in languages with freer word order such as German.
It is motivated by the desire to focus or emphasize the extraposed expression, or it serves to reduce center embedding and in doing so, it increases right-branching, right-branching structures being easier to process in English than left-branching structures: The crossing lines identifying the discontinuity are again easily visible.
In cases of it-extraposition, the optionality disappears; extraposition is obligatory: The star * indicates that the sentence is nonidiomatic.
Apparently when it appears in subject or object position, it forces the modifying expression to be extraposed.
Traditional dependency grammars (e.g. Lucien Tesnière's Structural Syntax[11] and Igor Mel'čuk's Meaning-Text Theory[12]) approach discontinuities much differently.
Other dependency grammars, in contrast, take linear and hierarchical order to be of equal importance.
The following trees illustrate the movement/copying type of approach to discontinuities: The constituent that idea is seen as being first generated in its canonical position to the right of the verb likes.
They both assume that there are no actual discontinuities in surface syntax (for there are no crossing lines in the trees).
What appears to be a discontinuity at first blush is actually rather an indication that movement or feature passing has occurred.