empty category

In linguistics, in the study of syntax, an empty category is a nominal element that does not have any phonological content and is therefore unpronounced. Empty categories may also be referred to as covert nouns, in contrast to overt nouns which are pronounced. The phenomenon was named by Noam Chomsky in his 1981 LGB framework. Some empty categories are governed by the empty category principle. When representing empty categories in trees, linguists use a null symbol to depict the idea that there is a mental category at the level being represented, even if the word(s) are being left out of overt speech. There are four main types of empty categories: NP-trace, Wh-trace, PRO, and pro. The types are differentiated by their two binding features: the anaphoric feature [a] and the pronominal feature [p]. The four possible combinations of plus or minus values for these features yield the four types of empty categories. Empty categories are present in most of the world's languages, although different languages allow for different categories to be empty.