In 1965, Christopher Alexander wrote A City Is Not a Tree, an essay on urban planning.
The type of tree he had in mind was not a biological tree with roots, leaves, and a trunk. Rather, he meant it in the sense that a mathematician or computer scientist would — with a tree being something like your computer’s file system. A tree in this sense is a conceptual structure. Trees have a single root node, which may have one or more “children”, which in turn might have more children, and so on.
Wikipedia gives the following visualization:
This makes trees good for representing “containment”, where the root node represents a container, and its children are contained within it, and their children contained within them, and so on. This is roughly how computer file systems work: a root directory contains files and directories, which can contain other files and directories, and so on.
We could think of the root of the tree as representing the city at the top level, districts of the city at the next level, and individual neighbourhoods below that. Perhaps another layer could represent particular amenities — schools, parks, hospitals — contained within the neighbourhood.
This is exactly the way that Alexander tells us not to think of cities.
The explanation is in the title — A City Is Not a Tree. Alexander argues that