A default order of syntactic components of a higher level unit, minimally a phrase, but more generally a clause or a sentence, is presupposed. It is also assumed that a movement rule may displace a syntactic component to some position different from its default position. Examples of relevant rules include ones which dislocate a topical constituent to sentence-initial position, which front a focal constituent to clause-initial position or which require the clause-initial position of a pronoun. Given these presuppositions, then a clause component left in situ (Latin for ‘in [its proper] place’) is one not so moved, but keeping its default position.

An example is the position of the interrogative pronoun in English content interrogatives. The default position of the direct object is following the verb, as in Linda bought a car. The content interrogative applies focus fronting to the interrogative pronoun, producing sentences like What did Linda buy? If this rule does not apply, the result is Linda bought what? The direct object is here left in situ.