In color display tubes (1) with a dotted shadow mask (13) structure and an in-line electron gun (10), the geometry of the screen (6) and the deflection field produced by the deflection unit (11) cause triad rotation, which is a rotation of the three phosphor dots (red, green and blue) corresponding to one aperture of the shadow mask (13) with respect to the horizontal scan lines (35). This problem can be solved by giving the horizontal lines (35) a curved shape. However, in the prior art situation a parabolic shape is used, leading to severe moiré problems, because the vertical pitch asv (38) is influenced too strongly at the vertical screen edges. This problem can be overcome by using fourth order or even sixth order terms in the shape of the horizontal lines (35). A pure fourth order function reduces the vertical pitch asv 38 variation by 50% with respect to the parabolic prior-art situation, and a pure sixth order function even leads to a reduction by 67%.