A pattern is a form, template, or model (or, more abstractly, a set of rules) which can be used to make or to generate things or parts of a thing, especially if the things that are generated have enough in common for the underlying pattern to be inferred or discerned, in which case the things are said to exhibit the pattern.

A Pattern Language is a structured method of describing good design practices within a particular domain. It is characterized by

1. Noticing and naming the common problems in a field of interest,
2. Describing the key characteristics of effective solutions for meeting some stated goal,
3. Helping the designer move from problem to problem in a logical way, and
4. Allowing for many different paths through the design process.

Pattern languages are used to formalize decision-making values whose effectiveness becomes obvious with experience but that are difficult to document and pass on to novices. They are also effective tools in structuring knowledge and understanding of fundamentally complex systems without forcing oversimplification -- including organizing people or groups involved in complex undertakings, revealing how their functions inter-relate as part of the larger whole.
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There is one timeless way of building. It is a thousand years old, and the same today as it has ever been. The great traditional buildings of the past, the villages and tents and temples in which man feels at home, have always been made by people who were very close to the center of this way. It is not possible to make great buildings, or great towns, beautiful places, places where you feel yourself, places where you feel alive, except by following this way. And, as you will see, this way will lead anyone who looks for it to buildings which are themselves as ancient in their form, as the trees and hills, and as our faces are."