Absolute rest-frame B-band magnitude (M_B) vs. flux ratio ℛ(6630/4400) at maximum light in 2D SN Ia models of Kasen et al. (2009) (small dots), and in data from the CfA SN Ia sample (A_V < 0.45 mag: filled circles, A_V > 0.45 mag: open circles). The dotted and dashed lines are linear fits to the models and data, respectively, where models for which ℛ(6630/4400) > 0.25 and data for which A_V > 0.45 mag have been excluded from the fit. The slope (Γ) and Pearson correlation coefficient (r) are indicated for both cases. The data have been offset vertically for clarity. Including models for which ℛ(6630/4400) < 0.25 results in Γ = 5.71 ± 0.03 and r = 0.97, while including data with A_V > 0.45 mag results in Γ = 4.43 ± 0.47 and r = 0.92. The arrows indicate approximate reddening vectors for different values of R_V.