In an anisotropic medium, such as a crystal, the polarisation field P is not necessarily aligned with the electric field of the light E. In a physical picture, this can be thought of as the dipoles induced in the medium by the electric field having certain preferred directions, related to the physical structure of the crystal. This can be written as:
Here χ is not a number as before but a tensor of rank 2, the electric susceptibility tensor. In terms of components in 3 dimensions:
or using the summation convention:
Since χ is a tensor, P is not necessarily colinear with E.
In nonmagnetic and transparent materials, χ = χ, i.e. the χ tensor is real and symmetric. In accordance with the spectral theorem, it is thus possible to diagonalise the tensor by choosing the appropriate set of coordinate axes, zeroing all components of the tensor except χ, χ and χ. This gives the set of relations:
The directions x, y and z are in this case known as the principal axes of the medium. Note that these axes will be orthogonal if all entries in the χ tensor are real, corresponding to a case in which the refractive index is real in all directions.
It follows that D and E are also related by a tensor:
Here ε is known as the relative permittivity tensor or dielectric tensor. Consequently, the refractive index of the medium must also be a tensor. Consider a light wave propagating along the z principal axis polarised such the electric field of the wave is parallel to the x-axis. The wave experiences a susceptibility χ and a permittivity ε. The refractive index is thus:
For a wave polarised in the y direction:
Thus these waves will see two different refractive indices and travel at different speeds. This phenomenon is known as birefringence and occurs in some common crystals such as calcite and quartz. 
If χ = χ ≠ χ, the crystal is known as uniaxial.  (See Optic axis of a crystal.) If χ ≠ χ and χ ≠ χ the crystal is called biaxial. A uniaxial crystal exhibits two refractive indices, an "ordinary" index (n) for light polarised in the x or y directions, and an "extraordinary" index (n) for polarisation in the z direction. A uniaxial crystal is "positive" if n > n and "negative" if n . Light polarised at some angle to the axes will experience a different phase velocity for different polarization components, and cannot be described by a single index of refraction. This is often depicted as an index ellipsoid.