The Robertson–Schrödinger relation gives the following uncertainty principle: σ σ L x σ σ L y ≥ ≥ ℏ ℏ 2 | ⟨ ⟨ L z ⟩ ⟩ | . where σ σ X is the standard deviation in the measured values of X and ⟨ ⟨ X ⟩ ⟩ denotes the expectation value of X. This inequality is also true if x, y, z are rearranged, or if L is replaced by J or S.