In the previous chapter we assumed that all the molecules were parallel to a fixed direction. This is the minimal energy configuration and it will be obtained if the nematic is free of external constraints. This is not always the case, as boundary conditions imposed by the container are not necessarily compatible with perfect alignment of the sample. An example is the case of a nematic between two glass plates with different molecular anchoring (e.g., planar and homeotropic), inducing bulk distortion of the director field. The resulting increase in free energy is the integral
of the elastic deformation free energy density f(n, ∇ n, ∇ 2n,...) over the entire nematic sample. In this chapter we describe this functional and its properties and show that, though fluid, a nematic can transmit torques.