Empty category

In linguistics, an empty category, which may also be referred to as a covert category, is an element in the study of syntax that does not have any phonological content and is therefore unpronounced. Empty categories exist in contrast to overt categories which are pronounced. When representing empty categories in tree structures, linguists use a null symbol to depict the idea that there is a mental category at the level being represented, even if the word are being left out of overt speech. The phenomenon was named and outlined by Noam Chomsky in his 1981 LGB framework, and serves to address apparent violations of locality of selection — there are different types of empty categories that each appear to account for locality violations in different environments. Empty categories are present in most of the world's languages, although different languages allow for different categories to be empty.

Null DPs

While the classical theory recognizes four types of null DPs, recent research has found evidence for null DPs that don't appear to fit the classical model such as the distinction of null subjects and null objects.

The classical theory

In the classical theory model, empty DPs can be broken down into four main types: DP-trace, WH-trace, PRO, and pro. Each appears in a specific environment, and is further differentiated by two binding features: the anaphoric feature and the pronominal feature . The four possible interactions of plus or minus values for these features yield the four types of null DPs.

		Symbol	Name of null DP	Corresponding overt noun type
+	+	PRO	"big Pro"	none
-	+	pro	"little Pro"	pronoun
+	-	t	DP-trace	anaphor
-	-	t	Wh-trace	R-expression

In the table, means that the particular element must be bound within its governing category. means that the empty category is taking the place of an overt pronoun. Having a negative value for a specific feature indicates that a particular type of null DP is not subject to the requirements of the feature.
Not all empty categories enter the derivation of a sentence at the same point. Both DP-trace and WH-trace, as well as all the null heads, are only generated as the result of movement operations. "-trace" refers to the position in the sentence that holds syntactic content in the deep structure, but that has undergone movement so that it is not present at the surface structure. Conversely, both "PRO" and "pro" are not the result of movement and must be generated in the deep structure. In both the government and binding and minimalism frameworks, the only method of base-generation is lexical insertion. This means that both "PRO" and "pro" are held to be entries in the mental lexicon, whereas DP-trace and Wh-trace, and null heads are not categories in the lexicon.

PRO (Big Pro)

The empty category subclass called PRO, referred to orally as "big pro", is a DP which appears in a caseless position. PRO is a universal lexical element, it is said to be able to occur in every language, in an environment with a non-finite embedded clause. However, its occurrence is limited: PRO must occupy the specifier position of the embedded, non-finite clause, such as in the example below:
This example does not use PRO, but instead, uses an overt pronoun in the specifier position of the embedded non-finite clause:
1a) He_i would like you_j to stay.
This example does use PRO, because instead of an overt pronoun, there is an empty category which is co-referenced with "He", appearing in the specifier position of the non-finite embedded clause:
1b) He_i would like PRO_i to stay.
The example tree to the right is the tree structure for this sentence, , and shows PRO surfacing in the specifier position of the TP in the embedded clause, and co-referenced to the subject of the matrix clause. We can interpret this as the DP subject having control over PRO. In other words, the meaning of PRO is determined by the meaning of DP , as they are co-referenced. This is an example of a subject control construction, where the pronominal subject is selected for by both the main verb and the embedded infinitive verb , thus forcing the introduction of an unpronounced lexical item at the subject of the embedded clause, in order to fulfil the selectional requirements of both verbs. Alternatively, we see object control constructions when the object of the sentence controls the meaning of PRO.
However, while the meaning of PRO can be determined by its controller, it does not have to be. PRO can either be controlled or uncontrolled. The realization that PRO does not behave exactly like an R-Expression, an anaphor, or a pronoun led to the conclusion that it must be a category in and of itself. It can sometimes be bound, is sometimes co-referenced in the sentence, and does not fit into binding theory.
Note that in modern theories, embedded clauses that introduce PRO as a subject are CPs.

''pro'' (little pro)

"Little pro" occurs in a subject position of a finite clause and has case. The DP is ‘dropped’ from a sentence if its reference can be recovered from the context; "pro" is the silent counterpart of an overt pronoun. Spanish is an example of a language with rich subject-verb morphology that can allow null subjects. The agreement-marking on the verb in Spanish allows the subject to be identified even if the subject is absent from the spoken form of the sentence. This does not happen in English because the agreement-markings in English are not sufficient for a listener to be able to deduce the meaning of a missing referent.
Chinese is an example of a pro-drop language, where both subjects and objects can be dropped from the pronounced part of finite sentences, and their meaning remains clear from the context. In pro-drop languages, the covert "pro" is allowed to replace all overt pronouns, resulting in the grammaticality of sentences that do not have a subject nor object that is overtly pronounced:
This example illustrates how a Chinese question might be asked with "Zhangsan" as the subject and "Lisi" as the object:
Below is an example of a response to the question above. Both subject and object are optionally pronounced categories. The meaning of the sentence can be easily recovered, even though the pronouns are dropped.
The same point can be made with overt pronouns in English, as in the sentence “John said I saw him”, where the chance of picking as the antecedent for is clearly greater than that of picking any other person.
In example 4), the null object must be referring to the matrix clause subject but not the embedded subject , since condition C of the Binding Theory states that it must be free.

DP-trace (t_DP)

In certain syntactic environments, case features are unable to be “checked”, and a determiner phrase must move throughout the phrase structure in order to check the case features. When this happens, a movement rule is initiated, and the structure is altered so that we hear the DP overtly pronounced in the position of the sentence which it has been moved to; a DP-trace is an empty category that appears at the original spot of the DP, and stands for the syntactic space in the tree that the DP previously occupied. DP-trace is found in complementary distribution to PRO.
Underlying word order in the sentence "Cheri seems to like Tony."
2a) seems Cheri to like Tony.
Spoken form of the sentence "Cheri seems to like Tony."
2b) Cheri seems to like Tony.
_{*Square brackets throughout example 2 indicate an empty DP category}
This English example shows that DP is originally introduced in the specifier position of the embedded infinitive clause, before moving to the specifier position of the matrix clause. This movement happens in order to check the features of the raising verb , and leaves behind a DP-trace in the original position of the DP. You can use the position of the DP-trace to identify where the DP is introduced in the underlying structure.

WH-trace (t_WH)

DPs can move for another reason: in the case of Wh-questions. In English, these are questions that begin with ; words that serve the same function in other languages do not necessarily begin with , but are still treated as “Wh-items” under this framework. The responses to these questions cannot be yes or no; they must be answered using informative phrases. Wh-items undergo Wh-movement to the specifier of CP, leaving a Wh-trace in its original position. Just like for DP-movement, this movement is the result of feature checking, this time, to check the feature in C.
To form a Wh-question in the example below, the DP moves to the specifier of the CP position, leaving a Wh-trace in its original position. Due to the extended projection principle, there is DP movement to the specifier of TP position. There is also T to C movement, with the addition of Do-support. These additional movement operations are not shown in the given example, for simplicity.
Example 5: Underlying order of words in the sentence “Who did Lucy see?”
5a) did Lucy see who
Spoken form of the sentence "Who did Lucy see?"
5b) Who did Lucy see ?
_{*Square brackets throughout example 5 indicate an empty category.}
_{*You can see where "Who" was in the initial word order by where the WH-Trace appears in the spoken form.}
The tree to the right illustrates this example of WH-trace. Initially, the sentence is " did Lucy see who,” which has an empty specifier position of CP, as indicated by square brackets. After the Wh-item is relocated to the specifier position of CP, the empty position is left at the end, in the original position of . What is left in its place is the WH-trace.
A special relationship holds between the WH-item and the complementizer of a sentence:
6a) is here.
6b) is here.
6c) * is here.
In this example, the complementizer or the WH-item can have null categories, and one or the other may show up as null. However, they cannot both be null when the WH-item is the subject.
An important note to remember is that DP-trace and WH-trace are the result of movement operations, while "pro" and "PRO" must be base generated.