According to the uncertainty principle of Heisenberg, if the two operators representing a pair of variables do not commute, then that pair of variables are mutually complementary, which means they cannot be simultaneously measured or known precisely. For example, the position and the linear momentum in the x -direction of a particle are represented by the operators x and − − i ℏ ℏ ∂ ∂ ∂ ∂ x , respectively (where ℏ ℏ is the reduced Planck constant). This is the same example except for the constant − − i ℏ ℏ , so again the operators do not commute and the physical meaning is that the position and linear momentum in a given direction are complementary.