Geometrical scheme of the non-LTE 2D-slab filament model and geometry of the solar_Y–solar_Z cross-section of the 2D slab in a large scale encompassing nearly half of the Sun shown in panels a and b, respectively. In order to distinguish the coordinate system of the slab itself from the helioprojective coordinate system (Cartesian coordinate system with origin in the Sun centre and its X-axis pointing to the solar W), the first one was denoted by simple X, Y, and Z letters while for the latter these three letters are preceded by the “solar_” prefix. Panel c: same geometrical scheme in more detail. While just a schematic cartoon explaining the 2D slab-model geometry is presented in the panel a, schemes in the panels b and c are produced as a graphical output of the model code. Size of the θ angle is exaggerated in all three panels for its better recognition, in reality θ is much smaller for the studied filament fragment.