When k relates to crystal momentum instead of true momentum, the concept of k -space is still meaningful and extremely useful, but it differs in several ways from the non-crystal k -space discussed above. For example, in a crystal's k -space, there is an infinite set of points called the reciprocal lattice which are "equivalent" to k = 0 (this is analogous to aliasing). Likewise, the " first Brillouin zone " is a finite volume of k -space, such that every possible k is "equivalent" to exactly one point in this region.