where "u" is periodic with the same periodicity as the crystal lattice. The actual quantum state of the electron is entirely determined by formula_2, not k or "u" directly. This is important because k and "u" are "not" unique. Specifically, if formula_2 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.