  The rigorous explanation for the term $| t |^{2\beta}$ in the rectilinear
diameter equation is given ($t = (T_c-T)/T_c$, $\beta$ is the critical exponent
for the asymptotic form of the equation of state). The optimal order parameter,
for which the branches of binodal are symmetric is constructed within the
canonical formalism. It is shown that the ratio of the amplitudes
$\f{D_{2\beta}}{D_{1-\alpha}}$ before $|t|^{2\beta}$ and $|t|^{1-\alpha}$ where
$\alpha$ determines the behavior of the heat capacity, takes the universal
character. The analysis of entropy for argon and water leads to $\beta = 0.33$
and $\f{D_{2-\beta}}{{D_{1-\alpha}}}\approx - 3.5$.
