Examples of empirical T_eff calibrations based on (a) the equivalent width of Mg I λ5167, (b) the equivalent-width ratio Fe II λ4583/Si II λ6347, and (c) the line-depth ratio He I λ6678/Mg II λ4481. Filled black symbols represent the reference-star measurements used to derive the calibration relationships. Open gray symbols, where shown, represent program-star measurements and are displayed only to illustrate the subsequent application of the relationships; they were not used in the fitting procedure. The solid curves show the best-fitting quadratic relationships derived from the reference-star sample. In the upper panels, the darker and lighter shaded regions indicate the propagated uncertainty of the fitted relationships and the total calibration uncertainty including the rms scatter, respectively. The lower panels show the normalized residuals, ΔT/σ; the darker and lighter shaded bands indicate the ±1σ and ±2σ intervals, respectively. The fitted equations, the numbers of reference stars used in the fits, and the rms scatter are given in the corresponding panels.