While relative entropy is a statistical distance, it is not a metric on the space of probability distributions, but instead it is a divergence. While metrics are symmetric and generalize linear distance, satisfying the triangle inequality, divergences are asymmetric in general and generalize squared distance, in some cases satisfying a generalized Pythagorean theorem. In general D KL ( P ∥ ∥ Q ) does not equal D KL ( Q ∥ ∥ P ) , and while this can be symmetrized (see § Symmetrised divergence), the asymmetry is an important part of the geometry.