The diagram showing the fusion of a mixture of 3 minerals A, B and C is the combination of the 3 binary diagrams A-B, B-C and C-A. Each shows a binary eutectic, the "coldest" point of the liquidus curve. The addition of the third mineral transforms these binary eutectic points into cotectic lines. Along these lines, the temperature decreases towards the inside of the triangle (in the direction of the arrows). The 3 cotectics meet at the "lowest" point of the diagram, the ternary eutectic where the melting reaction takes place: A + B + C = Liquid.

     To work on such a diagram, it is helpful to project onto the A-B-C triangle. As an example, we can see the fusion of the mantle at the origin of basaltic magmas .

     The dotted lines delineate isothermal planes.