This can be determined by drawing the isopleth from our starting composition to the base of the diagram. From X1 to X2 the liquid simply cools down to the liquidus surface. At X2, on the liquidus surface.
Plagioclase begins to crystallize.
This plagioclase has a composition represented by Pl2 and is in equilibrium with L2. The liquid composition moves down the liquidus from L2 to L3, becoming richer in the Ab component. Again note that the liquid composition is still more Ab-rich than the solid. In this type of system the horizontal lines from the liquidus to the solidus are isotherms, which join a liquid and solid phase which are in equilibrium with each other. In order to maintain equilibrium between the solid and liquid as the temperature drops the composition of the solid and liquid phases is constantly changing.