Themselves is a reflexive pronoun used in the English language. It refers back to a group of people or individuals previously mentioned, emphasizing that the action is performed by them. For example, in the sentence "They completed the project themselves," it highlights that the group did the work without assistance. In grammar, reflexive pronouns like themselves are essential for clarity and emphasis. They are often used to indicate that the subject and the object of a verb are the same. Other examples include myself, yourself, and ourselves, which serve similar functions in sentences.