In physics, complementarity is both a theoretical and an experimental result of quantum mechanics, also referred to as principle of complementarity, closely associated with the Copenhagen interpretation. It holds that objects have complementary properties which cannot be observed or measured at all at the same time.

The complementarity principle was formulated by Niels Bohr, a leading founder of quantum mechanics. Examples of complementary properties that Bohr considered:

Position and momentum
Energy and duration
Spin on different axis
Wave and particle
Value of a field and its change (at a certain position)
Entanglement and coherence

Bohr’s complementary principle should not be confused with observables that do not commute, like position and momentum. This is an entirely different subject.