Fig. 1. Average overproduction factors (over a Kroupa et al. (1993) IMF, see Eq. 4) of the yields of Woosley & Weaver (1995) for 3 different initial stellar metallicities, covering reasonably well the metallicity evolution of the Milky Way disk. The solid horizontal line is placed at and the two dotted horizontal lines at half and twice that value, respectively. Most of the intermediate mass elements are nicely co-produced (within a factor of two). The "odd-even effect" is clearly seen (e.g. in the cases of Na, Al, P, etc.); notice the small (but significant) metallicity dependence of the Ne and Mg yields. N behaves as a pure "secondary" element. The elements He, C, N, Li and Be obviously require another production site.