Empty category

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In syntax, an empty category is a nominal element which does not have any phonological content and is therefore unpronounced; they may also be referred to as covert nouns, in contrast to overt nouns which are prounounced. There are four types of empty category: NP-trace, Wh-trace, PRO, and pro. The types of empty category are differentiated by their two binding features, namely 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 category.

[a]	[p]	Symbol	Name of empty category	Corresponding overt noun type
-	-	t	Wh-trace	R-expression
-	+	pro	"little Pro"	pronoun
+	-	t	NP-trace	anaphor
+	+	PRO	"big Pro"	none

[edit] Generation of empty categories

Not all empty categories enter the derivation of a sentence at the same point. Both NP-trace and Wh-trace, as evidenced by their names, are only generated as the result of movement operations. Conversely, both PRO and pro are not the result of movement and must be base-generated. In both the GB and minimalism frameworks, the only method of base-generation is lexical insertion, so both PRO and pro are held to be entries in the mental lexicon.