Comparison of SN 20012Z and our N10def models with 0.01%, 0.22%, and 36% helium abundances. Optical and infrared spectra have been normalised to the median flux from 5000 Å–7000 Å and 10 000 Å –12 000 Å, respectively. The optical and infrared spectra of SN 2012Z are approximately +16 days and +17 days after B-band maximum and have been corrected for galactic and host extinction. Our N10def model is shown at +15 days after bolometric maximum light. Panels a and b: shaded regions represent He I λ5876, λ6678, λ7065, and λ10830 at rest and blue-shifted by the maximum velocity of the model (∼12 700 km s⁻¹). Panels c–d: zoom-ins of the regions surrounding He I λ5876, λ6678, λ7065, and λ10830, respectively. Rest wavelengths are denoted by a vertical dashed line.