Distribution of [N/Fe] versus [C/Fe] for metal-poor stars with observed carbon and nitrogen abundances. Star-shaped (red) symbols are stars with  [Fe/H]  <  −2.8, triangles (dark blue) have  −2.8 ≤  [Fe/H]  ≤  −1.8 and circles (yellow) have  [Fe/H]  >  −1.8. Giants (stars with log g ≤ 4) are shown as filled symbols while higher-gravity stars have open symbols. Large symbols represent data from Masseron et al. (2010) and the SAGA database; data from Lucatello et al. (2006) is shown with smaller symbols. Multiple data for the same star are connected with solid line segments. The dashed and solid lines indicate the criteria adopted for CEMP stars and NEMP stars, and the dash-dotted line is the alternative criterion  [(C + N)/Fe]  > 0.9 (see text).