In quantum physics, reciprocal space is closely related to momentum space according to the proportionality p = ℏ ℏ k , where p is the momentum vector and ℏ ℏ is the reduced Planck constant. The reciprocal lattice of a reciprocal lattice is equivalent to the original direct lattice, because the defining equations are symmetrical with respect to the vectors in real and reciprocal space. Mathematically, direct and reciprocal lattice vectors represent covariant and contravariant vectors, respectively.