Cartoon showing an example of two vectors v_fh located at fractions f = 1/2 (blue) and f = 3/4 (purple) of the maximum filament altitude h_max and the “base” vector v_b connecting the filament footpoints (black). The left plot shows the filament seen off-limb as a prominence, while the right plot shows the filament as seen from an observer located above the filament. If the higher part of the filament is rotated with respect to the position of the two footpoints, then the rotation angle α_fh can be determined as the angle between the vectors v_fh and v_b.