In linguistics and grammar, a pronoun (abbreviated PRO) is a word that substitutes for a noun or noun phrase. Pronouns have traditionally been regarded as one of the parts of speech, but some modern theorists would not consider them to form a single class, in view of the variety of functions they perform cross-linguistically. An example of a pronoun is "you", which is both plural and singular. Subtypes include personal and possessive pronouns, reflexive and reciprocal pronouns, demonstrative pronouns, relative and interrogative pronouns, and indefinite pronouns.