Left: the gradient of |$\mathrm{Si}\,\small {II}\,\lambda 6355$| velocity (km s^{− 1} d^{− 1}) as the function of B-band decline rate Δm₁₅(B). The sample and subclasses studied in Benetti et al. (2005) are overplotted. The sample is split into three different subclasses: ‘low velocity gradient’ (LVG; open squares), ‘high velocity gradient’ (HVG; open circles), and ‘FAINT’ (open triangles). Right: |$\mathrm{Si}\,\small {II}\,\lambda 5972$| pEW as the function of |$\mathrm{Si}\,\small {II}\,\lambda 6355$| pEW. The sample and subclasses studied in Branch, Dang & Baron (2009) are overplotted. The sample is split into four different subclasses: ‘shallow-silicon’ (open triangles), ‘core-normal’ (open circles), ‘cool’ (open downward triangles), and ‘broad-line’ (open squares). The position of SN 2013dy is represented by the red filled star. We also mark the positions of a variety of SNe Ia for comparison.