What does an expert software designer know? What knowledge went into building a software system? What do we need to teach people to become good designers or even to understand an existing design? These are important questions to software designers, but little progress has been made on discovering this knowledge and writing it down.

Patterns are a way of documenting design knowledge. Patterns make it easier to talk about designs, to evaluate tradeoffs, to understand existing designs, and to solve particular problems. They help train designers, help document complex systems, and help designers think and communicate. Better yet, patterns are a scheme to get people to document the knowledge that seems obvious to them, but profound to others.