ALFALFA mass-width function. Left panel: the colormap represents the mass-width function (MWF) measured by ALFALFA, i.e., the number density of galaxies within logarithmic bins of HI mass (M_HI) and velocity width (W). The measurement spans the range M_HI = 10⁷−10¹¹ M_⊙ and W = 28−900 km s^-1. Note that the stretch of the colormap is logarithmic. Right panel: colormap of the 1σ statistical error on the number density, expressed in dex (i.e., ). The plotted errors are just due to counting statistics, and systematic uncertainties are not included.