For ternary systems where actual isotherms are not known it is still possible to determine the dirctions that temperatures will fall by examining the relationship between tie lines and boundary curves.

Determining the directions of falling temperature
  1. Temperatures fall away from the three apices along the edges of the triangle.
  2. The point of intersection of a tie line joining two compounds, that share a boundary curve, with the boundary curve or an extension of the curve represents a thermal maximum along the pertinent section of the boundary curve.

These relationships can be illustrated by examing the hypothetical ABC system.

  1. The primary fields A + L and B + L share a boundary curve, and the tie line or Alkemade line joining the two solid phase is the AB side of the triangle, therefore the temperature along the boundary curve separating the A + L field from the B + L field must move away from the point of intersection along the boundary curve.
  2. The dashed line joining A and BC represents an Alkemade line, which crosses the boundary curve between the A + L and BC + L fields. Therefore the point of intersection is a temperature maximum, and temperatures fall away, in both direction, from the point of intersection.
  3. Compound BC is congruently melting, its bulk composition point lies within the primary phase field of BC + L.

This system exhibits two ternary eutectics, the arrows along the boundary curves converge at the eutectics.

Ternary peritectics occur in systems containing both congruent and incongruently melting compounds.

At ternary peritectics, the following relationships of boundary curves are possible:
  1. tributary - where two boundary curves come into the triple point and one goes out, or
  2. distributary - where one boundary curve comes into the triple point and two curves go out.