Ternary phase diagrams are 3 component systems. To construct a ternary diagram it is necessary to know the three binary systems for the three components. Ternary diagrams have a vertical temperature axis.

The actual ternary diagram may be represented as a three dimensional form or more commmonly as a two dimensional projection of the liquidus surface onto the base of the triangle created when the three binary diagrams are joined together. This irregular triangle is often transformed into an equililateral triangle to facilitate presentation and interpretation.

For experimentally studied ternary systems the liquidus surface may be contoured with the temperature interval representing the contour interval. The fields indicated on the ternary diagram represent the primary phase fields, of solid phases in equilibrium with liquid, present on the liquidus surface.

The lines or curves which separate primary phase fields, the red, blue and magneta lines on the above image, are referred to as Cotectic Lines or Boundary Curves, along which 3 phases are in equilibrium - two solids and a liquid.

Straight lines which join the composition points of two phases whose primary phase fields share a common boundary curve are called Alkemade Lines. Thus the edges of the triangle are Alkemade Lines as they join the individual phases AB, AC and BC, which share a boundary curve. Alkemade lines are a specific type of tie line.