The triple product rule, also known as the cyclic
chain rule, cyclic relation, cyclical rule or Euler's chain rule,
relates the partial derivatives of three interdependent variables,
and often finds application in thermodynamics. It is shown here
that its derivation is wrong, and that this rule is not correct; hence,
the Mayer's relation and the heat capacity ratio, which describe
the difference between isobaric and isochoric heat capacities, are
also untrue. Also, the relationship linking thermal expansion and
isothermal compressibility is wrong. These results are confirmed
by many experiments and by the previous theoretical findings of
the author.