A pattern presents a slice of the design space, which means
that it cannot -- or should not -- cover all the incidental
concerns or questions that may arise.
Alexander describes the use of patterns as the process of “differentiating space” where very high-level patterns are applied to an open space. As the space becomes more defined, lower-level patterns can be applied. This process is accomplished using a pattern language consisting of high-level patterns that are composed of dependent lower-level patterns.