a field of expertise. The term was coined by architect Christopher

Alexander and popularized by his book A Pattern Language. A pattern language is

an attempt to express the deeper wisdom of what brings aliveness within a

particular field of human endeavor, through a set of interconnected

expressions arising from that wisdom. Aliveness is one placeholder term for

"the quality that has no name": a sense of wholeness, spirit, or grace, that

while of varying form, is precise and empirically verifiable. Some advocates

of this design approach claim that ordinary people can use it to

successfully solve very large, complex design problems.

Like all languages, a pattern language has vocabulary, syntax, and grammar—but

a pattern language applies to some complex activity other than

communication. In pattern languages for design, the parts break down in this

way: The language description—the

"vocabulary"—is a collection of named, described solutions to problems in a

field of interest. These are called "design patterns." So, for example, the

language for architecture describes items like: settlements, buildings,

rooms, windows, latches, etc. Each solution includes "syntax," a

description that shows where the solution fits in a larger, more

comprehensive or more abstract design. This automatically links the solution

into a web of other needed solutions. For example, rooms have ways to get

light, and ways to get people in and out.

The solution includes "grammar" that describes how the solution solves a

problem or produces a benefit. So, if the benefit is unneeded, the solution is

not used. Perhaps that part of the design can be left empty to save money

or other resources; if people do not need to wait to enter a room, a simple

doorway can replace a waiting room. In the language description, grammar and

syntax cross index to other named solutions, so the designer can quickly

think from one solution to related, needed solutions, and document them in a

logical way. In Alexander's book, the patterns are in decreasing order by

size, with a separate alphabetic index. The web of relationships in the index of

the language provides many paths through the design process.

This simplifies the design work because designers can start the process from any

part of the problem they understand and work toward the unknown parts. At the

same time, if the pattern language has worked well for many projects, there is

reason to believe that even a designer who does not completely understand the

design problem at first will complete the design process, and the result will

be usable. For example, skiers coming inside must shed snow and store

equipment. The messy snow and boot cleaners should stay outside. The

equipment needs care, so the racks should be inside.

It really is a language: There is even an analogy to spelling or phonology, in

the documentation standards for the designs and patterns. Without these, the

people building the design won't be able to read the design.

What is a pattern? When a designer designs

something—whether a house, computer program, or lamp—they must make many

decisions about how to solve problems. A single problem is documented with its

typical place, and use with the most common and recognized good solution seen

in the wild, like the examples seen in dictionaries. Each such entry is a

single design pattern. Each pattern has a name, a descriptive entry, and some

cross-references, much like a dictionary entry. A documented pattern should

explain why that solution is good in the pattern's contexts.

Many patterns form a language Just as words must have grammatical and

semantic relationships to each other in order to make a spoken language useful,

design patterns must be related to each other in position and utility order to

form a pattern language. Alexander's work describes a process of

decomposition, in which the designer has a problem, selects a solution, then

discovers new, smaller problems resulting from the larger solution.

Occasionally, the smaller problems have no solution, and a different larger

solution must be selected. Eventually all of the remaining design problems are

small enough or routine enough to be solved by improvisation by the builders,

and the "design" is done. The actual organizational structure is

left to the discretion of the designer, depending on the problem. This

explicitly lets a designer explore a design, starting from some small part.

When this happens, it's common for a designer to realize that the problem is

actually part of a larger solution. At this point, the design almost always

becomes a better design. In the language, therefore, each pattern

has to indicate its relationships to other patterns and to the language as a

whole. This gives the designer using the language a great deal of guidance about

the related problems that must be solved.

The most difficult part of having an outside expert apply a pattern language

is in fact to get a reliable, complete list of the problems to be solved. Of

course, the people most familiar with the problems are the people that need a

design. So, Alexander famously advocated on-site improvisation by concerned,

empowered users, as a powerful way to form very workable large-scale initial

solutions, maximizing the utility of a design, and minimizing the design

rework. The desire to empower users of architecture was, in fact, what led

Alexander to undertake a pattern language project for architecture in the

first place. Design problems in a context

An important aspect of design patterns is to identify and document the key

ideas that make a good system different from a poor system, and to assist in the

design of future systems. The idea expressed in a pattern should be general

enough to be applied in very different systems within its context, but still

specific enough to give constructive guidance.

The range of situations in which the problems and solutions addressed in a

pattern apply is called its context. An important part in each pattern is to

describe this context. Examples can further illustrate how the pattern

applies to very different situation. For instance, Alexander's pattern "A

PLACE TO WAIT" addresses bus stops in the same way as waiting rooms in a

surgery, while still proposing helpful and constructive solutions. The

"Gang-of-Four" book by Gamma et al. proposes solutions that are independent

of the programming language, and the program's application domain.

Still, the problems and solutions described in a pattern can vary in their

level of abstraction and generality on the one side, and specificity on the

other side. In the end this depends on the author's preferences. However, even

a very abstract pattern will usually contain examples that are, by nature,

absolutely concrete and specific. Patterns can also vary in how far they

are proven in the real world. Alexander gives each pattern a rating by zero, one

or two stars, indicating how well they are proven in real-world examples. It is

generally claimed that all patterns need at least some existing real-world

examples. It is, however, conceivable to document yet unimplemented ideas in a

pattern-like format. The patterns in Alexander's book also

vary in their level of scale - some describing how to build a town or

neighbourhood, others dealing with individual buildings and the interior of

rooms. Alexander sees the low-scale artifacts as constructive elements of

the large-scale world, so they can be connected to a hierarchic network.

= Balancing of forces = A pattern must characterize the problems

that it is meant to solve, the context or situation where these problems arise,

and the conditions under which the proposed solutions can be recommended.

Often these problems arise from a conflict of different interests or

"forces". A pattern emerges as a dialogue that will then help to balance

the forces and finally make a decision. For instance, there could be a pattern

suggesting a wireless telephone. The forces would be the need to communicate,

and the need to get other things done at the same time. A very specific pattern

would be just "WIRELESS TELEPHONE". More general patterns would be "WIRELESS

DEVICE" or "SECONDARY ACTIVITY", suggesting that a secondary activity

should not interfere with other activities.

Though quite unspecific in its context, the forces in the "SECONDARY ACTIVITY"

pattern are very similar to those in "WIRELESS TELEPHONE". Thus, the

competing forces can be seen as part of the essence of a design concept

expressed in a pattern. = Patterns contain their own rationale =

Usually a pattern contains a rationale referring to some given values. For

Christopher Alexander, it is most important to think about the people who

will come in contact with a piece of architecture. One of his key values is

making these people feel more alive. He talks about the "quality without a

name". More generally, we could say that a good

system should be accepted, welcomed and happily embraced as an enrichment of

daily life by those who are meant to use it, or - even better - by all people it

affects. For instance, when discussing a street café, Alexander discusses the

possible desires of a guest, but also mentions people who just walk by.

The same thinking can be applied to technical devices such as telephones and

cars, to social structures like a team working on a project, or to the user

interface of a computer program. The qualities of a software system, for

instance, could be rated by observing whether users spend their time enjoying

or struggling with the system. By focusing on the impacts on human

life, we can identify patterns that are independent from changing technology,

and thus find "timeless quality". Generic structure and layout

Usually the author of a pattern language or collection chooses a generic

structure for all the patterns it contains, breaking each into generic

sections like context, problem statement, solution etc.

C. Alexander's patterns, for instance, each consist of a short name, a rating,

a sensitizing picture, the context description, the problem statement, a

longer part of text with examples and explanations, a solution statement, a

sketch and further references. This structure and layout is sometimes

referred to as the "Alexandrian form". Alexander uses a special text layout to

mark the different sections of his patterns. For instance, the problem

statement and the solution statement are printed in bold font, the latter is

always preceded by the "Therefore:" keyword. Some authors instead use

explicit labels, which creates some degree of redundancy.

= Meaningful names = When design is done by a team, pattern

names will form a vocabulary they can share. This makes it necessary for

pattern names to be easy to remember and highly descriptive. Some examples from

Alexander's works are WINDOW PLACE and A PLACE TO WAIT.

Aggregation in an associative network A pattern language, as thought by

Alexander, contains links from one pattern to another, so when trying to

apply one pattern in a project, a designer is pushed to other patterns

that are considered helpful in its context.

In Alexander's book, such links are collected in the "references" part, and

echoed in the linked pattern's "context" part - thus the overall structure is a

directed graph. A pattern that is linked to in the "references" usually addresses

a problem of lower scale, that is suggested as a part of the higher-scale

problem. For instance, the "PUBLIC OUTDOOR ROOM" pattern has a reference to

"STAIR SEATS". Even without the pattern description,

these links, along with meaningful names, carry a message: When building a

place outside where people can spend time, consider to surround it by stairs

where people can sit. If you are planning an office, consider to arrange

workspaces in small groups. Alexander argues that the connections in the

network can be considered even more meaningful than the text of the patterns

themselves. The links in Alexander's book clearly

result in a hierarchic network. Alexander draws a parallel to the

hierarchy of a grammar - that is one argument for him to speak of a pattern

language. The idea of linking is generally

accepted among pattern authors, though the semantic rationale behind the links

may vary. Some authors, however, like Gamma et al. in Design Patterns, make

only little use of pattern linking - possibly because it did not make that

much sense for their collection of patterns. In such a case we would speak

of a pattern catalogue rather than a pattern language.

= Usage = Alexander encouraged people who used his

system to expand his language with patterns of their own. In order to

enable this, his books do not focus strictly on architecture or civil

engineering; he also explains the general method of pattern languages. The

original concept for the book _A Pattern Language_ was that it would be published

in the form of a 3-ring binder, so that pages could easily be added later; this

proved impractical in publishing. The pattern language approach has been used

to document expertise in diverse fields. Some examples are architectural

patterns, computer science patterns, human computer interaction patterns,

educational patterns, social action patterns, and group facilitation

patterns. The pattern language approach has also been recommended as a way to

promote civic intelligence by helping to coordinate actions for diverse people

and communities who are working together on significant shared problems.

Alexander's specifications for using pattern languages as well as creating

new ones remain influential, and his books are referenced for style by

experts in unrelated fields. It is important to note that notations

such as UML or the flowchart symbol collection are not pattern languages.

They could more closely be compared to an alphabet: their symbols could be used

to document a pattern language, but they are not a language by themselves. A

recipe or other sequential set of steps to be followed, with only one correct

path from start to finish, is also not a pattern language. However, the process

of designing a new recipe might benefit from the use of a pattern language.

= Simple example of a pattern = Name: ChocolateChipRatio

Context: You are baking chocolate chip cookies in small batches for family and

friends Consider these patterns first:

SugarRatio, FlourRatio, EggRatio Problem: Determine the optimum ratio of