In the dark blue area where there is an abundance of water, everything is soluble to give but one phase. In the blue-green area, two phases form. A tie line through the overall composition intersects the envelope of this area at points that are the compositions of the two liquid phases. It is desirable to have the envelope as far from B and C as possible so that not much expensive polymer is needed to get the phases to form. However, the distribution coefficient may not be very favorable when the polymers are dilute.