"a Pattern Language" as a Network
"A Pattern Language" by Christopher Alexander1 is one of the most influential books in urban planning (as well as software development2).
The book models architectural and urban design as sets of "design patterns". Each pattern is composed of smaller patterns, and compose larger patterns in return, creating what is essentially a conceptual network of design elements.
Say, for example, we want to design a Seat in a Garden. This has it's own pattern in the book: Pattern 176 Garden Seat. Each pattern first describes how it fits into the larger patterns that it is part of:
"with the character of the garden fixed — garden growing wild (172), we consider the special corners which make the garden valuable and somewhat secret. Of these, the most important is the sunny place (161), which has already been described, because it is so fundamental to the building. Now we add to this another seat, more private, where a person can go to sit and think and dream. [...] see especially city country fingers (3) and quiet backs (59)"
It follows a more thorough description of what Alexander thinks makes for a good garden seat (and why it's important). Finally, he lists the patterns that are useful in constructing a garden seat:
"Place the garden seat, like other outdoor seats, where it commands a view, is in the sun, is sheltered from the wind — seat spots (241); perhaps under bushes and trees where light is soft and dappled — filtered light (238)"
Here are all 253 patterns connected in this way (I higlighted a minimal set of connections needed to keep all chapters connected3):
On Alexanders' steps to creating your design language
Alexander recommends picking most of the patterns directly related to the pattern one starts from, and after that to keep going as many steps as one wants - each time, "only include the patterns you especially like"1. He was aware how tightly knit his patterns are ("Your list can easily get too long"). The average number of connections per pattern4 is 14.77 - some patterns have a lot more connections than that. Here we see that most patterns have around 12 connections, but some have more than 30:
If we move four steps away from the largest pattern "independent regions", 49 out of the 253 patterns are involved. Below we show all patterns involved in "independent regions" if we move 4, 5 and six steps down the hierarchy: