This Demonstration shows a ternary phase diagram with a phase envelope. This diagram is used to represent the phase behavior of mixtures of three components that are only partially miscible over a range of compositions so that phase separation occurs. Select "diagram" and move the black dot to any location within the triangle by clicking on that location, which represents the overall composition of the mixture. Each corner of the triangle corresponds to a pure component. The mass fraction of a component in the mixture is read off the axis that is the same color as that component. These axes are labeled by drawing a line through the point; this line is parallel to the base of the triangle that is opposite the corner corresponding to a pure component. Select "phases" to view a diagram that shows the one- and two-phase regions.