While binary phase diagrams provide a useful introduction to the topic of phase relationships, most of the important research has done on ternary systems.
Ternary systems add an additional level of complexity.
Since there are now three components it is necessary to add an additional axis to the diagram to depict the system adequately, effectively a 3-D drawing. Unfortunately, 3-D diagrams are difficult to interpret. To overcome this problem we adopt a convention whereby the data is projected onto a two-dimensional plane with the temperature coordinate perpendicular to the surface of the diagram. The liquidus surface is then shown by a series of isotherms or lines of constant temperature. The result is not unlike a topographic map with contours for temperature rather than elevation.