Illustration of the substructure–triaxiality degeneracy: the figure shows 2D projected gas density isocontours of the simulated, average-mass relaxed cluster CL85, which we have picked here because its major axis is almost perpendicular to the viewing direction. Black: density isocontours after substructure removal for different choices. Blue dashed: original isocontours. While X = 3.5 does not remove all substructures, the stricter choice X = 2 also removes ellipticity from contours in the bulk within r₅₀₀.