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In Venn diagrams, sets (or concepts) are represented by a drawn boundary. Overlapping areas indicate where members of the overlapping sets have commonality.

Source: http://en.wikipedia.org/wiki/Image:British_Isles_Euler_diagram.svg

This is a very useful illustration of how a complex set of interlocking concepts can be made clear and understood quickly with a Venn (or Euler, or Johnstone) diagram - Euler diagram showing the components and classification of elements of the British Isles.

Legend:

Red: Geographic-only locations

Blue: Political entities (may also be geographic terms)