[Na/Fe] vs [Fe/H] for different stellar populations. The NSD stars are represented by green open and filled triangles, NSC stars by black stars, inner bulge stars by blue squares, solar neighbourhood thin-disc stars by red filled circles, and thick-disc stars by orange diamonds. Upper panels: trends from the individual spectral lines used in the analysis. Lower panel: mean trend. Not all three lines were usable for every star. The green open triangles (denoted ‘NSD Thin’) represent the abundances for the NSD stars determined using alternative stellar parameters with the assumption that oxygen abundances follow the thin-disc trend, indicating typical uncertainties.