Classical logic identifies a class of formal logics that have been most intensively studied and most widely used. The class is sometimes called standard logic as well. They are characterised by a number of properties:

1. Law of excluded middle and double negative elimination
2. Law of noncontradiction, and the principle of explosion
3. Monotonicity of entailment and idempotency of entailment
4. Commutativity of conjunction
5. De Morgan duality: every logical operator is dual to another