Binding theory deals with the relationships between expressions in a sentence that can refer to the same entities. It extends the traditional notion of pronouns and their antecedents to other categories. Binding theory is concerned with semantic dependencies between noun phrases, including the connection between a pronoun and its antecedent. It uses three word classes - referring expressions, anaphors, and pronominals - and principles to describe when different expressions may or may not be co-indexed and refer to the same entity.