The rock  R1 contains 67% plagioclase and 33% clinopyroxene. For simplicity, this is represented in a diagram by the pure poles (calcic and magnesian respectively) of these minerals.
      R1 is warmed up. When it reaches the solidus temperature, around 1275°C, it starts to melt, according to the eutectic reaction: Di + An = Liq.(=melt) in the proportion 40%An and 60% Di. The eutectic melt formed has a different composition to R1 and will always be the same regardless of the composition of the rock.
     As the proportion of minerals consumed by the reaction is different from that of the rock, the relative proportion of minerals in the rock will change: in the present case, the amount of Clinopyroxene decreases faster than that of plagioclase. (R1 moves, to the right, along the solidus curve ...). When all the clinopyroxene is consumed by the reactionv(...until the composition An=100%), the eutectic melting reaction stops due to the lack of clinopyroxene.
     If the temperature continues to rise, the anorthite melts and changes the composition of the magma, whose composition follows the liquidus curve.The proportion of anorthite crystals decreases until they disappear: The liquid L1 now has the same composition as the rock R1.
     The process is reversible and the cooling of L1 will result in the crystallisation of R1 by following the opposite path... unless the crystals are separated from the residual melt as they crystallise: this is the fractional crystallisation.