strength of the reference feature, [Si$\,$]{} $\lambda 6355$.

In the middle and right-hand panels of Figure \[f:sspec\], we show the Ca H&K feature and redder [Si$\,$]{} complex in detail. Again, it is clear that both $\mathcal{R}$(Si) and the strength of the [Si$\,$]{} $\lambda 3858$ feature change in the way described above.

A SN photosphere is, of course, more complicated than the simple [SYNOW]{} model. Specifically, as the temperature changes over the relevant range, the ionization of silicon (specifically the amount of singly and doubly ionized silicon) changes. Additionally, certain features may be saturated (and possibly for only certain temperatures). Specifically, it is thought that [Si$\,$]{} $\lambda
6355$ is usually saturated. However, it seems unlikely that the other [Si$\,$]{} features are saturated. Therefore, these additional complexities should not change our interpretation that the strengths of the [Si$\,$]{} $\lambda 3858$ and [Si$\,$]{} $\lambda 5972$ features are anti-correlated and change with temperature.

We fit the [Si$\,$]{} $\lambda 4130$, $\lambda 5972$, and $\lambda
6355$ features in each model spectrum with single Gaussians. Although the line profiles are not exactly Gaussian, the fits are reasonable approximations of the data, and the process is similar to what is done in practice. We also fit the Ca 