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 l_a and l_b along the horizontal tie line of the phase diagram are measured. The lever rule states that n_al_a = n_bl_b, where n_a is the amount of phase a and n_b is the amount of phase b. The rule takes its name from the similar form of the rule, m_al_a = m_bl_b, relating the moments of two masses m_a and m_b about a pivot in a lever.