Displacement is a ubiquitous phenomenon in natural languages. Grammarians often speak of displacement in cases where the rules for the canonical word order of a language lead to the expectation of finding a word or phrase in a particular position in the sentence whereas it surfaces instead in a different position and the canonical position remains empty: ‘Which book did you buy?’ is an example of displacement because the noun phrase ‘which book’, which acts as the grammatical object in the question, does not occur in the canonical object position, which in English is after the verb. Instead, it surfaces at the beginning of the sentence and the object position remains empty. Displacement is often used as a diagnostic for constituent structure because it affects only (but not all) constituents. In the clear cases, displaced constituents show properties associated with two distinct linear and hierarchical positions. Typically, one of these two positions c-commands the other and the displaced element is pronounced in the c-commanding position. Displacement also shows strong interactions with the path between the empty canonical position and the position where the element is pronounced: one often encounters morphological changes along this path and evidence for structural placement of the displaced constituent, as well as constraints on displacement induced by the path.
The exact scope of displacement as an analytically unified phenomenon varies from theory to theory. If more then one type of syntactic displacement is recognized, the question of the interaction between movement types arises. Displacement phenomena are extensively studied by syntacticians. Their enduring interest derives from the fact that the complex interactions between displacement and other aspects of syntax offer a powerful probe into the inner workings and architecture of the human syntactic faculty.