Theta role

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 1968) (Fillmore called theta roles "cases") and (Gruber 1965).

Theta roles are prominent in government and binding theory and the standard theory of transformational grammar.

The term "theta role" is often used interchangeably with the term thematic relations (particularly in mainstream generative grammar—for an exception see (Carnie 2006)). 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.

One common way of thinking about theta roles is that they are bundles of thematic relations associated with a particular argument position. (Carnie 2006)

Theta roles are stored in a verb's theta grid. Grids typically come in two forms. The simplest and easiest to type is written as an ordered list between angle brackets. The argument associated with the external argument position (which typically ends up being the subject in active sentences) is written first and underlined. The theta roles are named by the most prominent thematic relation that they contain. In this notation, the theta grid for a verb such as give is <agent, theme, goal>.

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