The word "enclosure" can be interpreted in two ways.
In a literal sense it is the spatial boundary between indoors and outdoors and is defined by the surfaces that separate the two. Those surfaces can be walls, ceilings and floors, and they can be made out of brick, wood, and glass to just name a few.
Another way of understanding enclosure is more psychological, and it is defined by degrees. A space can be completely enclosed with solid surfaces. It can be defined by a single plane that hovers above, or any number of variations. Some spaces demand more of a sense of enclosure. A bathroom, for examples, requires privacy, and so the number of windows may be reduced.
A space can be partially enclosed as well. A living room in a setting with a tremendous view and open space may want to have glass on all sides, but a living room in a dense urban environment may demand more enclosure for privacy.
Concerning the meeting of planes, here are some principles to keep in mind:
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