The translation vectors of the convenient unit cell, e.g. the cube edges of the fcc structure (which is not primitive) are used for the coordinate axes. The plane shown intercepts at 3, 2, 2 on the axes. As, by definition, these three points are not co-linear, therefore they can adequately define the plane. As we shall see later, the reciprocal of these numbers are more useful. First, reciproca...