was able to make such a measurement, and the polarization spectrum suggested that the blue component of the Ca H&K feature was from [Si$\,$]{} $\lambda 3858$ for SN 2001el.

Using the large CfA sample of SN Ia spectra [@Blondin12], @Foley11:vgrad determined that the velocity of [Si$\,$]{} $\lambda 6355$, , and the velocity of the red component of the Ca H&K feature, , at maximum light correlated with intrinsic colour, but did not correlate with light-curve shape (and thus luminosity). However, they did not find statistically significant evidence of a linear correlation between the pseudo-equivalent width of the Ca H&K feature and intrinsic colour. Using SDSS–II Supernova Survey and Supernova Legacy Survey data, @Foley12:vel confirmed these trends with high-redshift SNe Ia. They also noted a slight (2.4-$\sigma$ significant) trend between the maximum-light () and host-galaxy mass.

@Maguire12 [hereafter ] presented a sample of maximum-light low-redshift SN Ia spectra obtained with the *Hubble Space Telescope* (*HST*). After various quality cuts, the sample consisted of 16 spectra of 16 SNe Ia. These spectra covered the Ca H&K feature, but did not cover wavelengths near [Si$\,$]{} $\lambda 6355$. Unlike @Foley11:vgrad and @Foley12:vel, which presumed that the red component of the Ca H&K feature was 