The description of a classical system of F degrees of freedom may be stated in terms of a 2 F dimensional phase space, whose coordinate axes consist of the F generalized coordinates q i of the system, and its F generalized momenta p i. The microstate of such a system will be specified by a single point in the phase space. But for a system with a huge number of degrees of freedom its exact microstate usually is not important. So the phase space can be divided into cells of the size h 0 = Δ q i Δ p i, each treated as a microstate. Now the microstates are discrete and countable and the internal energy U has no longer an exact value but is between U and U + δU, with δ δ U ≪ ≪ U .