In the ternary system consisting of sodium sulfate and magnesium sulfate, the following solid phases appear in the temperature range from -10 to 110°C:
The temperatures and concentration ranges at which these solids appear are shown in the figure below to the left. All fields in the figure represent saturated solutions. The equilibrium lines and the experimental data in the diagram represent compositions and temperatures at which two solid phases are in equilibrium with the same solution. The water content of the solutions are not shown. It can be thought of as a third dimension in the diagram
The 85°C isotherm of this system is shown below to the right. Because the temperature is fixed, the composition of all three components of this ternary system can be displayed in a two-dimensional diagram. The green lines in the figure below are tie lines indicating phases and compositions in equilibrium with each other.