A compound subject is two or more individual noun phrases coordinated to form a single, longer noun phrase. Compound subjects cause many difficulties in the proper usage of grammatical agreement between the subject and other entities (verbs, pronouns, etc.). In reality, these issues are not specific to compound subjects as such, coming up equally as well with compound noun phrases of all sorts; but the problems are most acute with compound subjects because of the large number of types of agreement occurring with such subjects.