U-band spectroscopic variation spanning the range of common SN properties: Hubble residuals standardized by SALT2.4 light curve parameters, EW(Si II λ6355) and v(Si II λ6355) (left toright). Composite spectra were calculated after dividing the sample into four quartiles based on each property, which was repeated at three representative phase ranges (− 6, 0, 6) for each subset. Subset composites were drawn such that the line color becomes darker as the parameter value decreases. Dotted lines indicate U-band spectral-index subdivision boundaries.