Note that such a measure μ μ for which densities can be defined always exists, since one can take μ μ = 1 2 ( P + Q ) although in practice it will usually be one that in the context like counting measure for discrete distributions, or Lebesgue measure or a convenient variant thereof like Gaussian measure or the uniform measure on the sphere, Haar measure on a Lie group etc. for continuous distributions. The logarithms in these formulae are usually taken to base 2 if information is measured in units of bits, or to base e if information is measured in nats. Most formulas involving relative entropy hold regardless of the base of the logarithm.