will call the Si/Ca ratio. The range for $\mathcal{R}$(Si), from effectively zero (when [Si$\,$]{} $\lambda 5972$ is difficult to discern) to [$\sim\!\!$ ]{}0.3, is approximately the range seen by all SNe Ia except SN 1991bg-like objects [e.g., @Blondin12; @Silverman12:lc]. The Ca/Si ratio has a range of 1.2 to 2.3. The ratio is affected by both the strength of the [Si$\,$]{} $\lambda 3858$ absorption and the [Si$\,$]{} $\lambda 3858$ *emission*, which fills in some of the Ca H&K absorption. As noted above, the strength of the [Si$\,$]{} $\lambda
3858$ feature can have a large affect on the Ca H&K line profile, and even dominates for many temperatures.

Since [Si$\,$]{} $\lambda 5972$ and $\lambda 6355$ do not show significant velocity differences with temperature, are relatively free of contamination from other species, and $\mathcal{R}$(Si) is a good indication of light-curve shape, we can use it as a proxy for light-curve shape for our models. Figure \[f:si\_cavel\] shows measured with a single Gaussian as a function of $\mathcal{R}$(Si). The measured  decreases in amplitude with increasing $\mathcal{R}$(Si), which corresponds to decreasing stretch and luminosity.

![Velocities measured with a single-Gaussian fit to the Ca H&K feature as a function of $\mathcal{R}$(Si) for the [SYNOW]{} model spectra. 