SNeIa absolute magnitudes, MB, as a function of oxygen abundances, OHO3N2. Full and open symbols indicate objects with abundances estimated by
procedures (a) and (b), respectively. Triangles, squares, and circles indicate the method used to derive distances: Tully–Fisher, PNe Luminosity Function, or Tully–
Fisher and Cepheids, respectively. The color indicates the type of SNIa: normal (black), reddened (red), or subluminous (blue). Right: the same plot, but considering
only normal SNeIa. The green solid line is a second-order polynomial fit to the data. The purple solid line is a second-order polynomial fit to four metallicity bins (see
the text). Short-dashed purple lines represent the 1σ uncertainty to this fit. The blue and red horizontal solid lines provide the averaged value in the low- and highmetallicity
regimes, respectively, with their 1σ uncertainty shown with the pale blue and red areas.