A City is Not a Tree

Alexander, Christopher

I believe that a natural city has the organisation of a semilattice; but that when we organise a city artificially, we organise it as a tree.


A set is a collection of elements which for some reason we think of as belonging together.


Whenever we have a tree structure, it means that within this struc-ture no piece of any unit is ever connected to other units, except through the medium of that unit as a whole.


Each unit in each tree that I have described is the fixed, unchanging residue of some system in the living city. A house , for example, is the residue of the interactions between the members of a family, their emotions and their belongings. A freeway is the residue of movement and commercial exchange. But a tree contains only very few such units – so that in a tree-like city only a very few of its systems can have a physical counterpart.


Each unit in each tree that I have described is the fixed, unchanging residue of some system in the living city. A house , for example, is the residue of the interactions between the members of a family, their emotions and their belongings. A freeway is the residue of movement and commercial exchange. But a tree contains only very few such units – so that in a tree-like city only a very few of its systems can have a physical counterpart. Thousands of important systems have no physical counterpart.


certain groups have been emphasized by the physical units of the physical structure, why are just these the most irrelevant ones?


Another favourite concept of the CIAM theorists and others is the separation of recreation from everything else. This has crystallized in our real cities in the form of playgrounds. The playground, asphalted and fenced in, is nothing but a pictorial acknowledgment of the fact that 'play' exists as an isolated concept in our minds. It has nothing to do with the life of play itself. Few self-respecting children will even play in a playground.


When we think in terms of trees we are trading the humanity and richness of the living city for a conceptual simplicity which benefits only designers, planners, administrators and developers. Every time a piece of a city is torn out, and a tree made to replace the semilattice that was there before, the city takes a further step toward dissociation.


When we think in terms of trees we are trading the humanity and richness


When we think in terms of trees we are trading the humanity and richness of the living city for a conceptual simplicity which benefits only designers, planners, administrators and developers.


Every time a piece of a city is torn out, and a tree made to replace the semilattice that was there before, the city takes a further step toward dissociation.


One of the things I’ve often said about that project is that if we had more understanding and respect for the local building culture, and introduced innovations much more gradually in the context of it, we might still be building houses there, or an outgrowth of the project would still be in place.