Joint probability distributions (JPDs) of Ca II K line properties of fibrillar pixels and their background counterparts that have double-peaked profiles; panels a and b: JPD of the average intensity of the K_1V minima and the K₂ peaks in the fibrillar pixels and their backgrounds. The average intensity value of the K_1V and K₂ features are normalised to the local continuum intensity. The distribution of the K₂-peak asymmetry is shown on panel c. Panels d and e: K₁ and K₂ wavelength separations in the pixels of fibrils and their backgrounds; the bin width in these two panels is 78 mÅ that is, the spectral sampling size of CHROMIS around the Ca II K line core. The correlation between the quantities is represented by the Pearson correlation coefficient r on each panel. The y = x line is overplotted for comparison.