Lemmon's Theorem is a result in the field of mathematical logic, specifically in modal logic. It provides a framework for understanding the relationships between different modal systems, which are systems that deal with necessity and possibility. The theorem helps to clarify how certain axioms and rules can be used to derive conclusions about modal statements. The theorem is named after E. J. Lemmon, a prominent logician known for his work in modal logic and related areas. By establishing connections between various modal logics, Lemmon's Theorem aids in the analysis of how different logical systems can be compared and understood in a unified way.