A modal logic in which more than one primitive modal operator is employed. For example, formalizing the principle of Kant’s law that what is obligatory must also be possible typically involves a bimodal logic with a deontic operator $\text{O}$ (for ‘…is obligatory’) and an alethic modal operator $\square$ (for ‘…is necessary’). Multi-modal logics can have arbitrarily many modalities, e.g., an epistemic logic with a knowledge operator ${\text{K}}_m$ for each of $n$ many agents.