1. What or which person or persons: Who left?
2. Used as a relative pronoun to introduce a clause when the antecedent is a person or persons or one to whom personality is attributed: the visitor who came yesterday; our child, who is gifted; informed sources who denied the story.
3. The person or persons that; whoever: Who believes that will believe anything.
[Middle English, from Old English hwā; see kwo- in the Appendix of Indo-European roots.]
Usage Note: According to the traditional rule, who is a nominative pronoun (that is, it acts as the subject of a clause) and whom is an objective pronoun (that is, it acts as a grammatical object). Thus it's correct to say I like the actor who supports the governor, where the relative pronoun who is the subject of supports the governor, or Who supports the governor? where the interrogative pronoun who is the subject of supports the governor. Like other nominative pronouns, who can also serve as the complement of a linking verb, as in We learned who the governor's supporter is, where who is the complement of the linking verb is whose subject is the governor's supporter. In contrast, whom is correct in I despise the governor whom the actor supports and Whom does the actor support? where whom is the object of support, and the governor whom the actor campaigned for (or for whom the actor campaigned), where it is the object of the preposition for. Note that in all these cases, whom is used when it is the object of the verb or preposition, not when it merely comes after the verb or preposition. When the relative pronoun is the subject of an embedded clause, as in I wonder who supports the governor? or I know nothing about who supports the governor, who is correct and whom is an error, because in these instances it is the entire clause, not just the pronoun, that is the object of the verb wonder or the preposition about. · Despite the traditional grammatical distinctions outlined above, in practice whom is uncommon in speech and everyday writing because it has a formal tone. In informal contexts, who often replaces whom, as in Who does the actor support? or I despise the governor who the actor supports. (A common workaround for the problematic choice between formal whom and grammatically questionable who is to replace the relative pronoun with that, converting the governor whom the actor supports into the governor that the actor supports, or to omit it altogether, yielding the governor the actor supports.) Whom survives as the standard form when it is the grammatical object of a preposition that immediately precedes it, as in the governor for whom (not for who) the actor campaigned. · Some usage guides insist that who should be used only for humans, and that which or that must be used for animals, but that is not true when the animal is construed as similar to humans because it is given a name, considered as an individual, or credited with belief and volition. In our 2013 ballot, 76 percent of the Usage Panel accepted the use of who as a relative pronoun in The dogs who obeyed the commands got a treat, and the vast majority (93 percent) accepted it in My spaniel Molly, who is two years old, has just had a litter of puppies. See Usage Notes at else, that, whose.
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