The change in the regions defining pure hybrid homogeneous waves for the anisotropic guide with all-positive diagonal tensor elements as the angles of orientation are changed. Values used: ε_1s = 3.3856, ε_3s = 2.25, ε_1f = 4 and ε_3f = 2.7889. The blue areas show how changing the value of ϕ from ϕ = 0° (the blue curves) to ϕ = 90° (the red curves) has little effect on the range of θ over which pure hybrid homogeneous modes are possible.