If χ xx = χ yy ≠ χ zz, the crystal is known as uniaxial. (See Optic axis of a crystal.) If χ xx ≠ χ yy and χ yy ≠ χ zz the crystal is called biaxial. A uniaxial crystal exhibits two refractive indices, an "ordinary" index (n o) for light polarised in the x or y directions, and an "extraordinary" index (n e) for polarisation in the z direction. A uniaxial crystal is "positive" if n e > n o and "negative" if n e < n o. 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.