NPs can be overtly and non-overtly present in a sentence. Overt NPs are physically present and phonetically pronounced while non-overt NPs are not physically present and pronounced at the surface structure. Anaphor, pronominal and referential expressions are three types of overt NPs and their behavioral features in a sentence is regulated by binding mechanisms. Empty category is a cover term for PRO, pro, NP-trace and Wh-trace. All these are non-overt NPs and except PRO rest of the three empty categories share behavioral features with their overt counterparts. Pro behaves like pronominal, NP-trace has the same behavioral features as that of an anaphor and wh-trace behaves like referential expressions. Out of these four empty categories, PRO and pro are base generated and the other two are movement generated. PRO is the non-overt subject occurring at the subject position of non-finite clauses that is an ungoverned position. In English, it is also a case less position. PRO can have either controlled or arbitrary reference. A control PRO can either have a reference controlled by the subject of the matrix clause, known as subject control or it can be controlled by the object of the main clause, therefore the term, object control. Arbitrary PRO has a generic reference. Small pro is the non-overt subject pronoun occurring at the subject position of finite clauses in Pro-drop languages. The presence and absence of pro at the subject position of finite clause in a language is determined by the pro-drop parameter. English is not a pro-drop language. Traces are results of movement rule move-ɑ.When an element in syntactic structure moves from one syntactic position to the other through move-ɑ, it leaves behind traces. Traces can be of two types namely, NP-trace and Wh-trace. NP-trace is a result of DP-movement from one argument position to another. The trace that is left after that movement is called NP-trace. Wh-traces are results of wh-movement i.e. movement of wh- words to the spec of CP, that is movement of a maximal projection to a non-argument position. Wh- movements leave behind Wh-trace. Empty Category Principle governs two of the empty categories namely, NP-trace and Wh-trace. According to ECP, empty categories especially traces must be properly governed. This sums up empty categories.
INTRODUCTION TO LINGUISTICS & PHONETICS - EMPTY CATEGORY