The double-tangent construction of coexisting phases is an elegant approach to visualize all the multiphase binary systems that satisfy the equality of chemical potentials and to select the stable state. In this paper, we show how to perform the double-tangent construction of coexisting phases for binary systems modeled with the gamma-phi approach. By introducing the molar Gibbs energy change on mixing (gm), the mathematical equations to handle are simple and can easily be used by the students during exercise classes.