Active voice is a grammatical voice common in many of the world’s languages. It is the unmarked voice for clauses featuring a transitive verb in nominative–accusative languages, including English and most other Indo-European languages. Active voice is used in a clause whose subject expresses the agent of the main verb. That is, the subject does the action designated by the verb. A sentence whose agent is marked as grammatical subject is called an active sentence. In contrast, a sentence in which the subject has the role of patient or theme is named a passive sentence, and its verb is expressed in passive voice. Many languages have both an active and a passive voice; this allows for greater flexibility in sentence construction, as either the semantic agent or patient may take the syntactic role of subject.