Quantum mechanics provides two fundamental examples of the duality between position and momentum, the Heisenberg uncertainty principle Δ x Δ p ≥ ħ /2 stating that position and momentum cannot be simultaneously known to arbitrary precision, and the de Broglie relation p = ħ k which states the momentum and wavevector of a free particle are proportional to each other. In this context, when it is unambiguous, the terms " momentum " and "wavevector" are used interchangeably. However, the de Broglie relation is not true in a crystal.