In Freehand-Drawing we substitute Space by an increased cube's interior outfitted with quadratic subdivisions. The first cube's perspective sketch according to our chosen view-point should be created by heart. Further grid points might regularly be constructed through the Diagonal Method later.
     If the observer's picture plane intersects all planes of the target corner, a third Vanishing Point will appear. Vanishing points being off paper make the position of extreme oblique lines uncertain.
     We substitute the big cube sliding off our visual cone by a set of small cubes placed into the target corner. Thus the structure of interior space can be transformed into an object seen from outside. Earlier we moved inside of the big cube's interior, now, in order to be able to draw it from outside as an object, we diminish it by further subdividing its original structure.
     The first cube of the grid should be the heart-cube of the target corner. Its sketching might be supported with useful data taken from related orthogonal pictures.