In linguistics, a theta role or θ-role is the semantic role a noun phrase plays in a sentence. The term thematic role denotes the same concept. As such it is a semantic rather than a syntactic feature, in contrast to such notions as the subject of a sentence or a prepositional object.

For instance, in the sentence Debra broke the window, "Debra" is both the subject of the sentence and the agent and "the window" is the object of the verb and the patient. But in The window was broken by Debra, "Debra" is still the agent, even though "the window" is now the subject of the sentence.

Major theta roles

Here is a list of the major theta roles, using the example sentence, Debra broke the window with a bat and Jack fell asleep.

• The agent (A) is whoever is intentionally carrying out some action. In the first example sentence, Debra is the agent.
• The experiencer (S) is someone or a thing who experiences some state. Thus, in Jack fell asleep, Jack is the experiencer. This is because Jack is not an agent, in that he did not "fall himself asleep". One semantic test used to distinguish the two roles is to ask if "I promise to..." makes sense. For example, "I promise to break the window" is contrasted with "I promise to fall asleep". Another example includes "What did she do?" contrasting with "What happened to her?"; this is shown with the pair "What did she do?" — "She broke the window." which is contrasted with "What happened to her?" — "She fell asleep."
• The patient or theme (O) is whatever is acted on. Thus the window is the patient.
• The instrument is whatever is being used to perform the action; the bat is the instrument.

Other theta roles exist in the literature, but many tend to be more controversial.

Relationship of syntax to theta roles

In languages such as English which rely heavily on word order and use frequent passivization, identification of theta roles from merely syntactic clues is often impossible. In more heavily case-marked languages, however, more information is often encoded syntactically.

Many languages, for instance, have an instrumental case, which explicitly marks the instrument of a sentence. However, in such languages the instrumental case may have other uses (such as being governed by certain prepositions).

Although either the patient or agent can function as the subject of a sentence even in unmarked usages, in ergative-absolutive and tripartite languages the case marking of the "subject" differs depending on the type of verb used, in a way that tends to reflect the theta role it occupies.

Uniqueness

Generally, only one noun phrase can occupy a certain theta role in a sentence. This does not include conjunctions such as Bill and Ted went shopping, or I was attacked by cats and by dogs. But a sentence such as The car broke the window with its fender strongly implies for most speakers that the car is acting as an agent, because the car and the fender cannot both be instruments.

See also

• Morphosyntactic alignment
• Thematic role
• Case grammar
• Kāraka

Grammar