Only two general types of phase diagrams are needed to describe all of the phase behavior observed so far for three-component lipid mixtures containing cholesterol that show phase coexistence. In addition to cholesterol, the other two components are a lipid whose fully hydrated bilayers are in a gel phase (Lβ or Lβ′) at the measurement temperature (solid lipid), and a lipid whose fully hydrated bilayers are in a liquid-disordered phase (Lα) at the measurement temperature (fluid lipid). (a) The Type I phase diagram has two regions of macroscopic two-phase coexistence. At high cholesterol concentrations, crystals of cholesterol monohydrate coexist with the liquid-ordered phase, Lo. At lower cholesterol concentrations, Lα and Lβ phases coexist. Proposed tielines are shown. (b) The Type II phase diagram has three regions of macroscopic two-phase coexistence, and one region of three-phase coexistence. In addition to the same two-phase regions seen in the Type I diagrams, there is a large two-phase coexistence region where Lα + Lo phases coexist. In addition, a triangle of three-phase coexistence is also seen in which Lα Lo + Lβ phases coexist. Tielines are shown in all of the regions of two-phase coexistence.