In generative grammar, a theta role or θ-role is the formal device for representing syntactic argument structure—the number and type of noun phrases—required syntactically by a particular verb. For example, the verb put requires three arguments (i.e., it is trivalent). The formal mechanism for implementing a verb's argument structure is codified as theta roles. The verb put is said to "assign" three theta roles. This is coded in a theta grid associated with the lexical entry for the verb. The correspondence between the theta grid and the actual sentence is accomplished by means of a bijective filter on the grammar known as the theta criterion. Early conceptions of theta roles include (Fillmore called theta roles "cases") and . Theta roles are prominent in government and binding theory and the standard theory of transformational grammar. Thematic relations The term "theta role" is often used interchangeably with the term thematic relations (particularly in mainstream generative grammar—for an exception see ). The reason for this is simple: theta roles typically reference thematic relations. In particular, theta roles are often referred to by the most prominent thematic relation in them. For example, a common theta role is the primary or external argument. Typically, although not always, this theta role maps to a noun phrase which bears an agent thematic relation. As such, the theta role is called the "agent" theta role. This often leads to confusion between the two notions. The two concepts, however, can be distinguished in a number of ways. Thematic relations express the semantic relations that the entities denoted by the noun phrases bear towards the action or state denoted by the verb. By contrast, theta roles are a syntactic notion about the number, type and placement of obligatory arguments. For instance, in the sentence Fergus ate the kibble, the fact that there are two arguments (Fergus and the kibble), and Fergus must be capable of volition and of doing the action, and the kibble must be something that can be eaten is a fact about theta roles (the number and type of the argument). The actual semantic type of the argument is described by the thematic relation. Not all theoretical approaches use theta roles. Theta roles are largely limited to the Chomskyan versions of generative grammar and lexical functional grammar. Many other approaches, such as functional grammar and dependency grammar, refer to thematic relations directly without an intermediate step in