A rule enabling the relative amounts of two phases a and b, which are in equilibrium, to be found by a construction in a phase diagram. (For example, a can be gas and b can be liquid.) The distances la and lb along the horizontal tie line of the phase diagram are measured. The lever rule states that nala=nblb, where na is the amount of phase a and nb is the amount of phase b. The rule takes its name from the similar form of the rule, mala=mblb, relating the moments of two masses ma and mb about a pivot in a lever.