A Pattern Language
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# What is a pattern?
We understand patterns -- as explained by philosopher and urban designer Christopher Alexander 
-- to be tools that promote life and aliveness. They manifest in diverse ways without prescribing specific implementations.
Patterns describe the common core of successful solutions to problems that recur in comparable contexts, and in that sense represent demonstrated, practical knowledge. The complex interplay of context, problem, and solution is crucial. In a pattern, these three elements are never isolated from each other. In turn, each pattern is integrated with other patterns and unfolds its power through this mutual integration.
# An example
A pattern from the groundbreaking 1977 book, A Pattern Language: Towns, Building, Construction , reads: “Light on Two Sides of Every Room.”
It answers the question: What makes one space more alive than others? While one physical space may be different from another, the context – the space-within-a-building – may be comparable. Evidence shows that, given the choice, people prefer to be in spaces that have light coming in from two sides,
while spaces that have light from only one side remain unused and empty….If you build a room with light on one side only, you can be almost certain that you will be wasting your money. People will stay out of that room if they can possibly avoid it. (1995 edition: 811)
# What is a pattern language? The solution of one problem can influence the solution of other problems and even create new problems. Therefore, a pattern can never be seen in isolation. Each pattern is entangled with and complements others.
For example, the pattern above, “Light on Two Sides of Every Room,” refers to a so-called related pattern of building called "Wings Of Daylight.” Such related patterns highlight the multiple relationships that characterize any process.
All patterns for a specific design connected in this way constitute a pattern language. A pattern language is never complete but always evolving.