byphrases

In linguistics, a by-phrase (also written byphrase) is a prepositional phrase introduced by the preposition by. Its core function is to mark the agent in passive sentences, but the term is also used more broadly for by-phrases that indicate means, source, or other relations related to the action.

In passive constructions, the by-phrase identifies who or what performs the action. Example: "The novel was written by George Orwell." The agent may be animate or inanimate, and the agentive phrase is often optional, since passive sentences can omit the agent: "The novel was written."

Other uses and distinctions: By-phrases can also express means, manner, or source in non-passive clauses, though these uses often overlap with other prepositional phrases such as by means of, with, or via. Examples: "She traveled by train," "The report was circulated by email," "The door was opened by a key." The primary function, however, remains marking the encoder of the action in a passive clause.

Syntax and variation: The by-phrase is typically placed after the main verb in English, as in "The letter was written by her." It can be fronted for emphasis or in questions: "By whom was the letter written?" In many varieties, the by-phrase is optional and may be omitted when the agent is unknown or unimportant.

Cross-linguistic perspective: Other languages mark agency differently in passive constructions (for example, German uses von, French uses par). The English by-phrase is a common way to express agenthood, and the term appears in traditional grammar and modern syntactic descriptions as a standard label for this construction.