 The ice rule was introduced by Linus Pauling in 1935 to account for the residual entropy of ice that had been measured by William F. Giauque and J. W. Stout.The residual entropy, formula_30, of ice is given by the formula

where formula_32 is Boltzmann's constant, formula_33 is the number of oxygen atoms in the piece of ice, which is always taken to be large (the thermodynamic limit) and formula_34 is the number of configurations of the hydrogen atoms according to Pauling's ice rule.Without the ice rule we would have formula_35 since the number of hydrogen atoms is formula_36 and each hydrogen has two possible locations.