When the opening in the wall is small, it is this opening which is measured.
This is a central position of the smaller size.
The opening is thus more positive in form than the wall on which it is superposed: the form of the hole is outlined against the comparative formlessness of the wall. Moreover the actual limits of the slab-shaped wall are so far away from those of the hole that the immediate surroundings of the hole appears formless.’ (VDL, AS IV, 13)
When the opening becomes larger, at some point it looses its form. Attention starts to go to the peripheral elements and their measures.
Large opening, losing its form
Peripheral disposition of the smaller sizes.
‘If the hole gets larger in proportion to the wall, its sides begin to relate to those of the wall itself rather than to each other. The interface between the open and solid areas of wall attaches itself to the solid, so that the latter takes on the appearance of a frame around the hole: the right- and left-hand sections of wall appear as piers, and the top section as a lintel (fig. c). The slab-shaped mass of the wall therefore breaks down into three bar-shaped pieces, each of which is superposed on the form of the whole wall. The hole as such no longer has any form…’ (VDL, AS IV, 14)
Drawings from AS VI.12,13,14