From Christopher Alexander's Nature of Order:
"The purpose of the boundary which surrounds a center is two-fold. First, it focuses attention on the center and thus helps to produce the center. It does this by forming the field of force which creates and intensifies the center which is bounded. Second, it unites the center which is being bounded with the world beyond the boundary. For this to happen, the boundary must at the same time be distinct from the center being bounded, must keep the center distinct and separate from the world beyond it, and yet also have the capacity of uniting that center with the world beyond the boundary. Then the boundary both unites and separates. In both ways, the center that is bounded becomes more intense.
Boundaries do the complex work of surrounding, enclosing, separating, and connecting in various different geometric ways, but one vital feature is necessary in order to make the boundary work in any of these ways: the boundary needs to be of the same order of magnitude as the center which is being bounded. If the boundary is very much smaller than the thing being bounded, it can't do much to hold in or form the center. A two-inch border cannot hold a three-foot field. In a room, the boundary between floor and wall needs more than a six-inch molding - a wainscoting, 30 inches high, is more in scale with both. An effective boundary for the river Seine consists of roads, walls, paths, quays, trees, something almost as massive as the river itself. In general it is necessary to think of boundaries as very large."
Slides from lecture on Nature of Order.