Left panel: correlation of N/O with O/H in DLA systems (triangles: Pettini et al. 2008), Galactic halo stars (squares: Spite et al. 2005), and extragalactic H II regions (small dots). The data of H II regions are from the sources assembled by Pettini et al. (2008). The error bar in the bottom corner is for DLAs. The dashed line is the approximate plateau level seen in extragalactic H II regions. The large yellow dot corresponds to the solar abundances (Asplund et al. 2009). Right panel: correlation of C/O with O/H. The data of H II regions are from Garnett et al. (1995, 1997, 1999) and Kobulnicky et al. (1997). The Galactic values are from Fabbian et al. (2009a). The error bar in the bottom left-hand corner gives an indication of the typical uncertainty for halo stars.