Mean pitch angle (left panel), mean of the two innermost spiral segments (middle panel), and weighted mean pitch angle (right panel) as a function of the integer value of the revised numerical Hubble stage, for all the grand-design and multi-armed spirals in our sample. The running mean and standard deviation of the mean are shown for the barred (red) and non-barred (blue) galaxies separately, whenever more than one data point lies within the bin (i.e. when T ≤ 8 for barred galaxies and T ≤ 5 for non-barred ones). We have added small random offsets (≲0.3) to the T values in the x-axis (integers) to avoid point overlapping.