'A City is Not a Tree' , C. Alexander , 1965 , Republished in Human Identity in the Urban Environment. Ed. G. Bell and J. Tyrwhitt, Penguin 1992 .
What struck me as an architectural student above all else was that if lecturers were so cock- sure about how to design, why were our towns and cities built on the principles which they espoused, so mind-numbingly dull and dismal compared to cities which had developed with little professional help. It seemed to me then that those cities which had been most rationalised and organised were the most inhospitable and unfriendly.
This essay by Christopher Alexander provided a new way of looking at the city. Two simple, contrasting diagrams showed beyond doubt that it is relationships and not separateness that make cities, and it is a reason why I have spent my time since in urban design.
The tree structure at the top shows how in this century we have separated out different forms of human activity by constructing environments which segregate: if we are driving then we have nothing to do with pedestrians; if we are at work then we have no interest in home, and so on. The diagram below shows how similar elements can be grouped within a semi-lattice structure which allows a far greater number of linkages. 'We can see just how much more complex a semi-lattice can be than a tree in the following fact: a tree based on twenty elements can contain at most nineteen further subsets of the twenty, while a semi-lattice based on the same twenty elements can contain more than 1,000,000 different subsets. 'So that's why so much new stuff was dull! These diagrams remain just as relevant today.
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