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[[core-api-layer,Layer]]
= Layer =
A Polygene™ Application must consist of at least one layer. More layers are common, often dividing the application along the
common architectural diagrams used on whiteboards, perhaps with a UI layer at the top, followed by a service or application
layer, then with a domain layer and finally some persistence layer at the bottom.

Polygene™ enforces this layering by requiring the <<core-bootstrap-assembly>> to declare which layer uses which other layer. And
<<core-api-visibility>> rules define that layers below can not locate composites in layers above. Also, defining that
"Layer1 uses Layer2" and "Layer2 uses Layer3" does NOT imply that Layer1 has <<core-api-visibility>> to Layer3. If that
is wanted, then it must be declared explicitly.