Geological and experimental evidence indicate that the three-phase field, plagioclase-alkalifeldspar-liquid, may terminate in several different ways. The possible terminations have been developed from Schreinemakers' rules governing the disappearance of three-phase fields. In igncous rocks, these different terminations may arise from variations in the relative amounts of additional components in magmas, or from changes of total pressure, or from structural changes that effect the extent of solid solution in feldspar.The three-phase region originates from the intersection of the solidus and the feldspar solvus. The available evidence regarding this intersection is reviewed, as is the evidence for the existence and form of the boundary curve on the feldspar liquidus. The data are used to project a series of isobaric polythermal and isobaric isothermal diagrams for each possible termination. Subsequent discussions relate the theoretical arguments to the natural evidence, suggest the more probable geological environments of some of the terminations, and indicate characteristic features of each termination.It may be possible to identify the type of termination involved in the crystallization of some rocks. The necessary data are the amounts and compositions of both kinds of coexisting feldspars and of the feldspar components of the coexisting liquid at one or more stages of the crystallization process. Volcanic rocks are most suitable for such studies.The terminations explain many of the compositional relationships possible between feldspar crystals and silicate melt under geological and experimental conditions. These compositional relationships are fundamental to understanding the crystallization of igneous rocks and the formation of melts by partial fusion. Other geological applications include a simple explanation for some resorbed feldspars, the separation, correlation, and comparison of porphyritic rock units, interpretation of compositional changes of successive zones of zoned feldspars, and mantling of one feldspar by another. With all types of termination, extensive fractionation yields a liquid rich in Or and Ab that may crystallize to alkali feldspar or feldspars.