Illustration of method used to test the separation in the age-metallicity map. Left: age-metallicity map of Nissen’s data (blue dots) with the division line for two populations (black dashed line) and with d_z parameter axes (black solid line). The solar twins projected across the d_z axis (colored points). Upper right: random distribution of 600 stars. Lower right: projection of the data from the upper right age-metallicity map to the d_z axis.