Claim: Linear grammar, a context-free grammar that has at most one nonterminal in the right-hand side of each of its productions, indicates that all linear languages are context-free and that regular languages are a proper subset of the linear languages.
Evidence: In computer science, a linear grammar is a context-free grammar that has at most one nonterminal in the right-hand side of each of its productions. All linear languages are context-free; conversely, an example of a context-free, non-linear language is the Dyck language of well-balanced bracket pairs. Hence, the regular languages are a proper subset of the linear languages, which in turn are a proper subset of the context-free languages.