The principle that for any theorem in electrical circuit analysis there is a dual theorem in which one replaces quantities with dual quantities; current and voltage, impedance and admittance, and meshes and nodes are examples of dual quantities.

The principle that one can obtain new solutions of Maxwell's equations from known solutions by replacing E with H, H with -E, ∊ with μ, and μ with ∊.

The principle that analogies may be drawn between a transistor circuit and the corresponding vacuum tube circuit.

A principle that if a theorem is true, it remains true if each object and operation is replaced by its dual; important in projective geometry and Boolean algebra.

See <<wave-particle duality>>