In the case of an ideal gas, the measure formula_9 is given by the Maxwell–Boltzmann distribution. It is a product measure, in that if formula_46 is the probability of atom formula_47 having position and velocity formula_48, then, for formula_40 atoms, the probability is the product of formula_40 of these. This measure is understood to apply to the ensemble. So, for example, one of the possible boxes in the ensemble has all of the atoms on one side of the box. One can compute the likelihood of this, in the Maxwell–Boltzmann measure. It will be enormously tiny, of order formula_51 Of all possible boxes in the ensemble, this is a ridiculously small fraction.