A biaxial nematic is a spatially homogeneous liquid crystal with three distinct optical axes. This is to be contrasted to a simple nematic, which has a single preferred axis, around which the system is rotationally symmetric. The symmetry group of a biaxial nematic is i.e. that of a rectangular right parallelepiped, having 3 orthogonal axes and three orthogonal mirror planes. In a frame co-aligned with optical axes the second rank order parameter tensor, the so-called Q tensor of a biaxial nematic has the form