There are two operators in classical modal logic:
- Necessity, represented by $\Box$ and
- Possibility, represented by $\Diamond$.
Modal logic may also have other operators, including:
- Temporal logic, which uses several operators including present and future;
- Epistemic logic, which uses operators "an individual knows that" and "for all an individual knows it might be true that";
- Multi-Modal logic, which uses more than two unary modal operators.
- Results about modal logic can be found here.