Lever Rule
Index
Shown is a partial binary phase diagram for the Copper-Nickel system with the alloy Cu-35 wt% Ni at the vertical line. At the point, a, the alloy is a liquid.  Upon cooling it passes through the two-phase (a + L) zone  and then solidifies as a single phase substitutional alloy.  Between (b) where the composition line intersects the liquidus line and (d) where it intersects the solidus line, the system exists as two phases in equilibrium. 

The Lever Rule permits the quantity of each phase present to be determined. If C0 is the mean composition of the alloy and Ca and CL are the points at which an isotherm cuts the solidus and liquidus lines, respectively, then the weight in the a-phase is given by:
Wa = (C0 - CL)/(Ca - CL), and the weight in the L-phase is:     WL = (Ca - C0)/(Ca - CL) . The mean composition acts as a "fulcrum" for the isotherm "lever" and the weight fraction of a phase is the length of the opposite lever arm over the total length of the arm.
From: Callister, 
"Materials Science and Engineering," 
Wiley (1994)