Semilattice

This diagram is an important part of Christopher Alexander's theories about good design, specifically urban design. His early essay "A City is not a Tree" asserts that a good city is instead a semi-lattice. The tree diagram is hierarchical and the points (places) are not connected. The semi-lattice has more interconnection. The points on such a diagram could be the public elements of a city (newspaper stand, bus stop, corner store, bench, house door, etc).

Christopher Alexander was firstly a mathmetician. He turned to design, specifically urban design and architecture secondly, so he approaches design problems from an atypical perspective.