In grammar, the voice (also called diathesis and (rarely) gender (of verbs)) of a verb describes the relationship between the action (or state) that the verb expresses and the participants identified by its arguments (subject, object, etc.). When the subject is the agent or doer of the action, the verb is in the active voice. When the subject is the patient, target or undergoer of the action, the verb is said to be in the passive voice. For example, in the sentence: The cat ate the mouse. the verb "ate" is in the active voice. However, in the sentence: The mouse was eaten by the cat. the verbal phrase "was eaten" is passive. In the sentence: The hunter killed the bear. the verb "killed" is in the active voice, and the doer of the action is the "hunter". A passive version of the sentence is: The bear was killed by the hunter. where the verbal phrase "was killed" is followed by the word "by" and then by the doer "hunter". In a transformation from an active-voice clause to an equivalent passive-voice construction, the subject and the direct object switch grammatical roles. The direct object gets promoted to subject, and the subject demoted to an (optional) complement. In the examples above, the mouse serves as the direct object in the active-voice version, but becomes the subject in the passive version. The subject of the active-voice version, the cat, becomes part of a prepositional phrase in the passive version of the sentence, and can be left out entirely.