Toomre diagrams for our WD sample. The two panels show the Galactic population classification based on S_tot (P. E. Nissen 2004; left) and the kinematic criteria of T. Bensby et al. (2003, 2005; right). The gray curves represent iso-total-velocity contours, while the blue and purple curves denote the velocity boundaries adopted to distinguish the thick- and thin-disk populations, respectively.