Top panel: I-band absolute magnitude on day 50 versus plateau duration. The plateau length stays approximately constant over a large range of magnitudes, though perhaps getting shorter for brighter SNe. Bottom panel: photospheric velocities on day 50 versus plateau duration. Since there is an established correlation between luminosity and velocity for SNe II-P, the plot resembles the one in the top panel. The red dots are a subset of grid models computed by Dessart et al. (2010), interpolated to satisfy the mass–velocity relation found by Poznanski (2013). These models show very good agreement with our measurements.