Figure 3.

The lever rule is the solution of the conservation of matter equations for the fraction of L_o phase (φ^o) and L_d phase (φ^d) and when the coexistence composition (ξ) and the end-point compositions (ξ^d and ξ^o) are all constrained to lie on a line (i.e. tie-line). The lever rule is essentially the parameterization of a tie-line, with the fractions of phase lying on the interval [0,1]. The lever rule is invariant under our ξ, to ψ coordinate transformation and has the form of a ratio of two Euclidean distances (x/z and y/z).