The five most important three-phase reactions in binary phase diagrams are the following:
| Eutectic | L → α + β |  | |
| Eutectoid | γ → α + β |  | |
| Monotectic | α + L → β |  | miscibility gap |
| Peritectic | α + β → γ |  | |
| Peritectoid | L1 → L2 + α |  | |
| symbol | description |
| α | solid |
| β | solid |
| γ | solid |
| L1 | liquid |
| L2 | liquid |
Many combinations of two elements produce more complicated phase diagrams than isomorphous systems. These systems contain reactions that involve three separate phases.
The horizontal lines indicate the presence of a three-phase reaction, and represents the temperature at which the reaction occurs under equilibrium conditions. There are three points at which lines intersect along the horizontal line. The point located near the center of the line represents the composition at which the three-phase reaction occurs.
α, β, γ, L1, and L2 all represent different phases.