This paper elaborates the postulate that the Earth and Moon became a binary system during their accretional development and that the Moon’s growth was essentially completed before the assumed solar nebula dissipated. The solar nebula was still hot enough at the formation of the two bodies that both consisted largely of the refractory and relatively low-density minerals now characteristic of the Moon. During the subsequent condensation, agglomeration and accretion of siderophile and more volatile higher density minerals, the Earth grew very much faster than the Moon because of (a) its much greater gravitational capture area coupled with retention by a sizeable atmosphere and (t>) the Moon’s velocity, with respect to the solar nebula, which produced a wind that aerodynamically blew away volatiles and smaller debris resulting from hypervelocity impacts of larger planetesimals. This ‘impact differentiation’ process favored the retention of the refractory minerals on the Moon (Figure 1). The Moon’s surprisingly high moment of inertia follows naturally from the basic postulate.