Metallicity variations for a range of spectral subtypes, shown in two different wavelength regions. The top panel shows the region around the Ca II H and K lines (∼3940 Å), which are metallicity sensitive for the higher-temperature stars (A6, F8). The bottom panel shows the region around the sodium doublet (∼5900 Å), which is metallicity sensitive for the spectral subclasses around F through early M-type stars. The equivalent widths of this particular sodium doublet (Na I D) are smaller for late-type M stars, making it difficult to use the feature to distinguish metallicity. The templates of the same spectral classes have no added constants to the flux, but the templates of different spectral classes have an added constant for readability.