When a state is measured, it is projected onto an eigenstate in the basis of the relevant observable. For example, if a particle's position is measured, then the state amounts to a position eigenstate. This means that the state is not a momentum eigenstate, however, but rather it can be represented as a sum of multiple momentum basis eigenstates. In other words, the momentum must be less precise. This precision may be quantified by the standard deviations, σ σ x = ⟨ ⟨ x ^ ^ 2 ⟩ ⟩ − − ⟨ ⟨ x ^ ^ ⟩ ⟩ 2 σ σ p = ⟨ ⟨ p ^ ^ 2 ⟩ ⟩ − − ⟨ ⟨ p ^ ^ ⟩ ⟩ 2 .