All Opening systems have dual character: an inside and an outside one. But very often a third facet is recognized as the depth of the opening (cross section of the opening). Here the significant elements are: the spatial thickness or the duration of transiting through the opening system, and the configuration of the opening system. The Depth of the opening is revealed in the bevelled edges or chamfered sides on inside or outside. On outer face it enlarges the size of the opening and on the inside face it diffuses the daylight illumination. A bottom taper brings the light to the floor and a sloped interior head illuminates the ceiling.
The third facet or the depth dimension of the opening systems is also the threshold. A threshold has two distinct worlds on either of the sides, one or both of which could be real or notional. A threshold could also be factual or abstract depending on the real or notional worlds across it. The threshold areas are resting zones, zone for transition, point of decision making, celebration, welcome or separation.
The third facet or the depth also governs the changes occurring in transit through the opening. The interim space or time changes like delay in passage, filtration, funneling, release, mixing, directional alignment, etc. change the quality of interior space. Some of these changes are passive, but many use energy. Active change enforcing elements are like window air-conditioner, air cooler, exhaust fans.
Depth of the door or windows regulates the pattern of illumination and field of view. The depth on exterior space enhances the shadows and so the depth, but the same on an interior side adds to the surface area of the space.
External walls of the buildings, till about the Gothic period were heavy offering two choices for showing the depth on ether the external or internal side. But Gothic buildings’ thinner walls did not such a play. So instead the Windows were elaborately divisioned. The trend reached its extreme with glass curtain wall buildings. Here the third dimension of the opening was completely eliminated. Mies van der Rohe was criticized for using very emphatic mullions.