Venn / Euler / Johnston diagrams
In Venn diagrams, sets (or concepts) are represented by a drawn boundary. Overlapping areas indicate where members of the overlapping sets have commonality.
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.
Red: Geographic-only locations
Blue: Political entities (may also be geographic terms)