Pitch angle versus H I gas fraction (upper panels) and H I surface density (lower panels) for S⁴G galaxies with inclination < 65°, including barred and non-barred and all types of spirals (multi-armed, flocculent, and grand-design). The Spearman’s correlation coefficient and significance are indicated on top of the plots. Different spiral types are shown with different colours and symbols, as indicated in the legend. The red line shows a linear fit to the cloud of points.