Alexander coined the term, pattern language, as a method of describing good design practices or patterns of useful organization within a field of expertise. Patterns help us remember and communicate insights and knowledge about design and can be used in combinations to create solutions, i.e. forms that fit.
It works like this.
There are hundreds of thousands of species of plants on Earth. However, we can categorize them into six plant type patterns: trees, palms, vines, shrubs, grasses, and forbs/herbs. There are all kinds of nuances, but this is all we need to know to put together a basic design template for a median.
There are certain universalities for trees. They are perennial, woody plants often with a single stem (trunk) and usually of a certain height. If we want a bit more nuance, then we drill down to the next level. We take some tens of thousands of different tree species and categorize them into seven growth form patterns:
And even though each species is unique, there are varying patterns that can be distilled from their structure, behavior, needs, services and yields to add further nuance. We can, and have begun to, organize these into various categories based on how they look and function (see The Elements [link to page]).
These categories are what we refer to as patterns. The cataloging of these provides us with a pattern language, a menu of forms that we can choose from to assemble a foodscape for a median responsive to its situational and geographic context.