We find a concise relation between the moduli $\tau , \rho$ of a rational Narain lattice $\Gamma (\tau, \rho)$ and the corresponding momentum lattices of left and right chiral algebras via the Gauss product. As a byproduct, we find an identity which counts the cardinality of a certain double coset space defined for isometries between the discriminant forms of rank two lattices.