The beginning of melting of mineral assemblages of alkali feldspar, plagioclase and quartz in the system Ab-Or-Q-An cannot be treated in terms of eutectic points in subsystems which are defined on the basis of the ratio Ab/An. Such subsystems do not exhibit unique melting temperatures or unique compositions of the first liquid produced. It is shown theoretically and demonstrated experimentally that these three minerals may coexist with liquid over a considerable portion of the melting range.