Comparison between the median spectra of SNe II and IIb in different time ranges: 0–10 days post-explosion, 10–20 days, 20–30 days and 30–40 days. Blue spectra correspond to SNe IIb, and red spectra to SNe II. The dashed vertical line marks the rest wavelength of He I (λ5876), while the continuous line marks the rest wavelength of Hα. The shaded regions represent the Median Absolute Deviation (MAD), which represents dispersion around the median.