Suppose an electron is in a Bloch state ψ ψ ( r ) = e i k ⋅ ⋅ r u ( r ) , where u is periodic with the same periodicity as the crystal lattice. The actual quantum state of the electron is entirely determined by ψ ψ , not k or u directly. This is important because k and u are not unique. Specifically, if ψ ψ can be written as above using k, it can also be written using (k + K), where K is any reciprocal lattice vector (see figure at right). Therefore, wave vectors that differ by a reciprocal lattice vector are equivalent, in the sense that they characterize the same set of Bloch states.